This, like the adventitious substance last described, is found as a subserous formation, on the outer side, and in the substance of the serous coat, after it has undergone a fibroid condensation of its tissue; and it also occurs in fibroid exudations upon the inner surface of the membrane. Its usual form is that of nodulated plates or cords of various size and thickness. It appeared to Meckel to be the result of an endeavor to convert a membranous into an osseous cavity, similar to that which prevails in the vertebral and cranial cavities. The granular and stalactitic form is less common; but both are sometimes found together at the same spot. Lastly, some shapeless concretions are met with, which are the chalky residue of fibrinous effusions.

The frequency with which serous membrane becomes the seat of ossification is a matter of much variety, depending on the different frequency of the changes of texture which precede it. Ossification occurs chiefly on the pleura, where it is remarkable for its extent and thickness. In the peritoneum it is almost confined to certain investing portions, especially to that covering the spleen; it occurs in the tunica vaginalis of the testicle, and, in the synovial system, sometimes in bursae.