* If there are cases which appear exceptions to this rule, such as those of the Manalus borealis of Fleming and the Albacore of Couch, they are instances in which not only the species, but even the genus, is as yet unascertained.
* It may be here observed, that a single species has been inserted in the body of the work as British, which perhaps is hardly entitled to be considered in that light. The author alludes to the Balcsna Physalus, Linn., of the occurrence of which in our seas, he can find no certain instance on record. He also entertains some doubts respecting the Spams Aurata, from the circumstance of its having been so often confounded with the S. ccntrodovtus.
Some other matters remain now to be spoken of. It has been already intimated that the descriptions are as far as possible original. It may be added, that in a large number of instances they are derived from recent specimens. It must be obvious, however, that in preparing a work of this nature, there will always be many species, especially among the large marine animals, which it is impossible for any individual to describe from his own observation. In such cases, then, recourse has been had to what have appeared the best authorities, more especially to such persons as have published any thing original on the species in question; and where under such circumstances the accounts of authors differ, the discrepancies are pointed out. It is believed that in almost all the above cases the name of the author, from whom any thing is borrowed, is subjoined*, who of course is responsible for the accuracy of what is stated. Desmarest and Scoresby (the last for the Cetaceous Animals) are the authorities mostly resorted to in the Class Mammalia; Temminck in that of Birds; Cuvier and Valenciennes, as well as Bloch, and occasionally Pennant and Donovan, in that of Fish. By some it may be thought that the descriptions are too long, and run needlessly into detail. But when it is considered how many species have been overlooked from their supposed identity with others; how many, some even of the most common occurrence, have been misunderstood, and referred to others which perhaps are not inhabitants of this country; and that these and similar errors have arisen not merely from the imperfect, but, it must be added, careless descriptions which have been given of such animals, it is hoped that the pains which have been taken to render this portion of the work as complete and accurate as possible, will not be thought entirely thrown away. The plan which the author has adopted in most instances, in the case of the Fish especially, which of all our British Ver-tebrata have till lately been the least attended to, and, in regard to the distinction of species, the worst understood, has been to describe as minutely as possible the first species in each genus, or sub-genus if it present a marked modification of form, and then, when there were other nearly allied species, to restrict himself principally to the differences which were observable in these last, with reference to the first and to one another. By this method, which is adopted from MM. Cuvier and Valenciennes, the enquirer is more readily enabled to identify the particular species he may have before him. In the Class of Birds, the different variations of plumage, arising from age and season, have been pointed out and characterized so far as they are known; and this plan has been pursued even in the case of those species which have occurred in this country as yet only in the immature state, for the more complete guidance of our own naturalists, in the event of their being met with in the adult plumage.
* A few instances occur in the Class Mammalia, in which this authority was omitted to be annexed. The cases in question are those of the Extirpated Quadrupeds, the Common and Great Seals, the Walrus, the Red Deer, the Roe, and two or three of the Cetacea, the descriptions of which have been borrowed, partly from Desmarest, and partly from other sources. There are also three species of Birds similarly circumstanced; the Shore Lark, the Rock Ptarmigan, and the Virginian Partridge. The description of the first is taken from Temminck and Wilson; that of the second from Sabine, and the "Fauna Boreali-Ame-ricana;" that of the third from Temminck.
What has been said of the descriptions applies also to the measurements. They are to be considered as original, excepting where the name of any author stands attached to them. They have been taken with much care, in by far the larger number of instances, from recently killed specimens, and many of them are the mean results obtained from measuring a large number of individuals. In the Class of Birds, they are generally those of the male, unless stated to the contrary. It may be observed that in this Class the entire length is measured from the tip of the bill to the extremity of the tail; the length of the bill is estimated in two directions, firstly from the frontal feathers, following the curvature of the ridge, secondly from the gape or angle of the mouth; the length of the tail is measured from the extremity of the longest quill to its insertion in the coccyx.
Fish being very variable in size, and having no very well marked limit of growth, it was thought that the absolute dimensions of any species, would, if given, prove of little value. Hence in this Class, the author has generally restricted himself to noting the average length to which the species attains; the relative proportions, which are often of great importance, being introduced into the body of the description. With reference to these proportions, it may be stated, that the entire length, unless mentioned to the contrary, is always measured to the extremity of the caudal fin. The length of the head is measured from the end of the snout to the posterior margin of the gill-cover. The depth (termed the breadth in the case of the Pleuro-nectidce) is reckoned vertically from the most elevated point in the line of the back, termed the dorsal line, to the corresponding point in the line of the abdomen, or ventral line, the dorsal and anal fins, unless mentioned to the contrary, not being included.