ON THE ANATO:\[Y OF FISHES. 
113 
Group — Pimelodixa . 
Platijstoma tigrinum. 
We have been able to examine two examples of this interesting South American 
Ibrm, one, a skeleton, about 2^ feet in length, now in the Zoological Museum of the 
Mason College, and the other a spirit-preserved specimen about 15 inches long. Our 
description of the skeleton applies more particularly to the larger of the two 
examples. 
The series of rigidly interconnected anterior vertebrae includes the first, the 
complex and the fifth and sixth vertebrae, but the articulation of these vertebrae with 
the skull is of a somewhat more flexible character than is the case with most other 
Siluridae. As compared with those of the normal free vertebrae which succeed them, 
the centra of the modified vertebrae, with the exception of that belonging to the first 
vertebra, are all greatly elongated, but less in girth, and more compressed than the 
former. 
The centrum of the first vertebra (figs. 31 and 32, v.') is exceptionally well 
developed, being nearly as large as any of the free centra, from which, however, it 
differs in the greater depth of its posterior concavity as compared with the shallow 
anterior concavity. It has no trace of transverse or of accessory articular processes. 
The complex centrum (c.c.) is greatly elongated and compressed laterally. Its 
anterior concavity is funnel-shaped, and the posterior almost tubular through its 
unusual depth. In the dry skeleton a cleft extends from the floor of the neural canal 
to about the centre of the anterior third of the complex centrum, and partially 
separates the anterior from the posterior tubular concavity (fig. 31); superiorly the 
cleft is continuous with a large foramen in the adjacent neural arch, through which 
the dorsal and ventral roots of the fourth spinal nerve emerge from the neural canal 
{sp.n.'^). We can hardly doubt that this cleft results from the imperfect coalescence 
of the bodies of the third and fourth vertebrae, and serves to indicate the line of union 
of the anterior concavity of the latter with the posterior concavity of the former. 
Neither the basioccipital nor the complex centrum develop accessory articular pro- 
cesses, but remain widely separated by the well-developed body of the first vertebra. 
The centrum of the fifth vertebra is a trifle longer than the complex centrum, but, 
like the latter, is much compressed ; the condition of its anterior and posterior 
surfaces is, however, reversed, the anterior being deep and tubular, and the latter 
funnel -.shaped and relatively shallow (fig. 31, v.^). The centrum of the sixth vertebra 
is much shorter, although longer than the normal centra ; anteriorly it is somewhat 
contracted, but widening behind it equals in the size the body of the seventh 
vertebra with which it articulates, like its predecessor it has the anterior cavity deeper 
than the posterior (r.®). The seventh vertebra (/.’.") is normal and free. Although 
closely and firmly articulated together, no anchylosis takes place between the neural 
MDCCCXClll. — n. ii 
