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PROFESSORS T. W. BRIDGE AND A. C. HADDON 
IV. — Morphological Summary. 
In summarising the more noteworthy of the results of our investigations into the 
morphology of the air-bladder and Weberian ossicles, and the correlated modifications 
which the anterior vertebrae and their processes undergo, we may indicate, in the 
first instance, such features as appear to be common to nearly all Siluroids, and 
secondly, those that are characteristic of particular genera or species. 
Although only demonstrated in one particular instance (the young of Amiurus 
catus) by Ramsay Wright, our researches lead us to 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 vertebrae 
indistinguishably combined to form an apparently single vertebra, for which we have 
ventured to suggest the name of “complex vertebra.” The discovery 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 in 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 presence of two pairs of nutrient foramiua on the ventral 
surface of its centrum. Nevertheless the constancy of its characters in nearly all 
Siluridse and the suppression of all trace of the second vertebral centrum as a distinct 
element, justifles the conclusion that the complex vertebra has the same value as in 
Amiurus. 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 much smaller than any of the 
normal centra. 
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 rudimentary. Unless represented by 
the horizontal processes of the intercalarum in some Siluridae (e.g., Macrones) and the 
tripodes, respectively, the second and third vertebrae are always devoid of transverse 
processes. The transverse processes of the fourth vertebra, on the conti’ary, are 
always greatly expanded, not Infrequently divided into anterior and posterior divisions 
