1889.] 



Contributions to the Anatomy of Fishes. 



311 



believe that the great majority of Siluroids agree with Amiurus in 

 having the centrum of the second vertebra, and the centra, neural 

 arches, and spinous processes of the third and fourth vertebras indistin- 

 guishably combined to form an apparently single vertebra, for which 

 we venture to suggest the name of " complex vertebra." The dis- 

 covery by Baudelot that the " complex vertebra " of the Cyprinidae was 

 formed by the fusion of the second vertebral centrum with the third 

 vertebra, was due to the distinctness of these elements in one particular 

 species, but no evidence of a similar nature is available m any but 

 embryonic Siluridae. In no adult Siluroid is there the slightest trace 

 of intervertebral spaces or sutures between the three confluent centra, 

 in fact, the only features which in any way suggest the composite 

 nature of the complex vertebra in that family are the perforation of 

 its neural arch by two pairs of spinal nerves and the occasional pre- 

 sence of two pairs of nutrient foramina on the ventral surface of its 

 centrum. This fusion of vertebrae in the formation of the " complex" 

 is almost invariably attended by the partial anchylosis of the latter to 

 the fifth vertebra, partly as the result of the firm sutural union of 

 their correlated elements, and in part due to the investment of the 

 lateral surfaces of their centra by a continuous deposit of superficial 

 bone. Moreover, the conjoined vertebrae, with the addition of the 

 centrum of the first, are so articulated to the skull that little, if any, 

 motion is possible, either between the individual vertebrae or between 

 the latter and the skull. The centrum of the first vertebra is nearly 

 always more or less rudimentary. 



With the possible exception of the claustra no distinct or ossified 

 intercalary elements are ever present. 



The first vertebra very rarely has transverse processes, and even 

 when present (e.g., some species of Arius) they are extremely rudi- 

 mentary. The transverse processes of the fourth vertebra, on the 

 contrary, are always greatly expanded, not infrequently divided into 

 anterior and posterior division by a cleft, and with or without the 

 aid of those belonging to the fifth vertebra form a more or less com- 

 plete investment to the dorsal and anterior walls of the air-bladder. 

 The sixth is, as a rule, the first rib-bearing vertebra ; exceptionally, 

 however, it may be the fifth (Gallichrous), or even the seventh 

 ( Glarias) . 



In almost all cases, except where they are modified to form an 

 "elastic-spring-apparatus," the transverse processes of the fourth 

 vertebra, in addition to their characteristic relations to the air-bladder, 

 form a more or less rigid support to the proximal elements of the 

 pectoral girdle. 



Over a somewhat triangular area, on each side, between the ex- 

 occipital in front and the anterior margin of the arch of the complex 

 vertebra behind, the wall of the neural canal is formed only by fibrous 



