ON THE ANATOMY OF FISHES. 
167 
“elastic-spring” mechanism to the walls of that organ. We regret that we have 
only been able to examine a skeleton of this species, but we venture to give a brief 
description of certain features in its structure for the sake of comparison with other 
types. It is interesting to remark that in many of its skeletal details Synodontis 
exhibits a much closer resemblance to the East Indian genus Pangasius {e.g., P. 
Buchanani, P. djanihal) than to those South American Siluroids in which an 
“elastic-spring” apparatus is present. 
The first, the complex (fig. 67, c.c.), and the fifth {v}), and sixth {v.^) vertebrae are 
rigidly connected together by investing superficial ossifications which continuously 
thicken the ventro -lateral surfaces of their centra, and terminate in front at the 
anterior end of the complex centrum in strong oblique lateral ridges that overlap the 
basioccipital, or rather articulate with its thickened ventral portion by an interdigi- 
tating suture. The true body of the first vertebra is therefore not visible externally 
on the ventral surface. There is a deep aortic groove (a.g.). Stout radial nodules 
(r.71.) are attached to the oblique lateral ridges of the complex centrum, and fairly 
broad dorsal laminae (dJ.) extend therefrom ventrad to the cardinal grooves, and 
blend with the ventral surfaces of the posterior divisions of the transverse processes 
of the fourth v^ertebra.' Unlike Aucheniptei'us, Doras, and Oxydoras, each of these 
processes in Synodontis is deeply cleft into anterior and posterior divisions, of which 
the latter [tpdp.) is directed obliquely backwards and acutely pointed at its free 
distal extremity, while the former is modified to form an “ elastic spring ” mechanism. 
The anterior division (tp.hi.) has an oblique origin from the arch of the complex ver- 
tebra, continuous behind with the flat but much stouter root of the posterior division, 
and, after extending slightly forwards, and, at the same time, curving downwards, 
becomes bent backwards on itself, and expanded into a flattened and somewhat semi- 
circular plate, the posterior surface of which is directed backwards and a little 
upwards (fig. 67). The anterior face of the plate is strengthened by a stout projecting 
process which extends forwards and upwards over the inferior limb of the post- 
temporal (pt.i.), and probably serves for the insertion of the protractor muscle of the 
mechanism. From their position and the direction of their faces these plates must 
have been applied to the lateral portions of the anterior wall rather than to the 
antero -lateral or lateral walls of tlie anterior chamber as is the case with Aucheni- 
jJteritsP The transverse processes of the fifth vertebra {tp:’) articulate with the basal 
portions of the posterior divisions of the preceding processes by irregular inter- 
digitating sutures, but otherwise form long triangular structures, directed obliquely 
* With regard to the terminal plates of the mechanism and their relations to the wall of the anterior 
chamber, it will be seen from the following extract from Sorknsen’s more recent paper (37, p. 140), that 
Nj/nodoniis closely agrees with Doras ; — “Chez le genre Synodontis, le disqne pen 4pais du ressort est 
nne ossification de la couche exteme de la membrane externe ; la couche interne est ici assez mince ot 
s’etend le long de la face interne (posterieure) dn disqne, dont elle est separee par nne masse epaisse 
gelatineuse d’une natnre particiiliere.” 
