h. Middle lobe of the liver, which is trilobed in this species, cut short. The right and left lobes are seen on either side of the aperture made in the abdominal walls.

i. Gall-bladder, in the fissure between the right and central lobes. The letters h and i are placed over the coracoidal bar.

k. Stomach, which has the form of a siphon. Blood-vessels are seen arising from the concave edge of the organ and uniting to form a vessel which passes towards the liver and is one of the factors of the portal system.

l. The spleen. m. The intestine with its anterior wall removed to show the spiral valve formed by its mucous membrane. Such a valve is found in all Elasmobranchii, in Ganoidei (rudimentary in Lepidosteus) and Dipnoi. The inclination, etc, of the folds varies much in the different genera.

n. Rectum. The line points to the spot where the rectal gland, found in all Elasmobranchii, opens into it on its dorsal aspect.

o. Dilated duodenum or Bursa Entiana. It receives the bile and pancreatic ducts. The pyloric aperture is placed on a nipple-like projection.

p. The line points to the fold which lies dorsal to the anal aperture into the cloaca, and separates it from a recess into which open the two oviducts laterally and the urethral canal medianly. In the male there is a urogenital papilla on the dorsal wall of the cloaca.

q. Right porus abdominalis or external aperture of one of the two canals by which the abdominal cavity communicates with the exterior in Elasmobranchii, Dipnoi, some Ganoidei, and a few Teleostei.

r. The two-lobed ventral fins. In a male the claspers would be situated between the inner lobes of these fins and the root of the tail.

s. Tail cut short.

FIG. 2. Heart, with ductus Cuvieri and base of the ventral aorta of Raja Batis. The walls of the several sections of the heart have been removed in part to show points of internal structure. Ventral view. From a specimen.

a. Ventricle, single as in all Pisces, Amphibia and Reptilia, with the exception of the Crocodile. The walls are thick, and the inner surface shows numerous bands of muscular tissue (columnae carneae). The cavity is curved and the entrance into it from the auricle is guarded by two large membranous valves, seen in the right upper corner of the diagram, - the left corner in the natural position.

b. Conus or bulbus arteriosus. The aperture of the ventricle leading into it lies on the opposite side to the auricular aperture. Its walls are thick, composed of striated muscular tissue, and it is rhythmically contractile. Its cavity contains three longitudinal rows of pocket-shaped membranous valves, four in each row. The distal valve in each row is the largest. The number of valves in each row appear to vary in the different species of Rays. The conus must be regarded as a part of the ventricle. It is present in Ganoidei and Dipnoi among Pisces, and in Amphibia. The number of rows of valves and of valves in each row that it contains is very variable. The distal valves persist and form the valves that guard the entrance to the aorta in those Vertebrata in which the conus is not present as a separate division of the heart.

c. The line points to the left posterior innominate artery. It and its homologue on the other side of the body spring from the base of the median aorta. The aorta is composed chiefly of membranous tissue, and the muscular tissue present in its walls is unstriped. (Cf. Fig. I, b and c).

d. d. The right and left pouches of the large thin-walled auricle. This structure is single as in all Pisces except Dipnoi. Its walls are thin and its muscles form a network of trabeculae.

e. e. The right and left ductus Cuvieri. These two vessels bring back to the heart the venous blood of the whole body. They fuse in the middle line posteriorly to the auricle forming the sinus venosus. The aperture of this sinus into the auricle is guarded by two membranous valves. The cavity of the whole heart makes an S-shaped curve, much more distinct in some Sharks than in the Ray. This curve is an embryonic feature in other Vertebrata.

FIG. 3. Brain, with the roots of the chief nerves of Raja Batis. Dorsal view. From a specimen.

a. The Lobi olfactorii connected by long olfactory tracts to the cerebral lobes from which they are outgrowths. Each lobe is lengthened out laterally, corresponding with the elongated nasal pit. In some Rays they are secondarily lobed. The olfactory tracts vary much in length in the Elasmobranchii. They are said to be wanting in Raja miraletus.

b. The cerebral lobes or main part of the fore-brain. They are much compressed dorso-ventrally, triangular in shape, with the outer angles swollen where the olfactory tracts take origin. They are slightly notched in front and grooved ventrally, indicating their bilaterally symmetrical structure. Lateral ventricles are wanting, but the degree to which they are obliterated among Elasmobranchii is very variable. A triangular spot, shaded in the diagram, behind the cerebral lobes marks the position of the third ventricle. The filamentous pineal gland (Epiphysis cerebri) has been removed.

c. The mid-brain, corpora bigemina, or optic lobes. These bodies are large and hollow, and lie above the aquaeductus Sylvii or passage between the third and fourth ventricles, with which their cavities communicate.

d. The cerebellum, or roof of the anterior portion of the fourth ventricle, the ventricle of the hind-brain. It is large in size in all Elasmobranch fishes, and often complexly convoluted. It consists here of two lobes in front and a long triangular lobe behind. It contains a large cavity, freely open to the fourth ventricle, which is just visible behind it, being for the most part covered by the posterior triangular lobe.

e. The convoluted corpora restiformia with which are connected the roots of the fifth, facial, and auditory nerves.

f. The commencement of the spinal cord. The medulla oblongata or the sides and floor of the hind-brain is remarkably short in the Rays. It is usually elongated in Elasmobranchii.