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Profs. T. W. Bridge and A. C. Haddon. [June 20, 



side of the vertebral column a broad, wing-like plate of bone, the 

 anterior margin of which is strongly decurved, for the investment of 

 the dorsal and anterior walls of the anterior chamber of the air- 

 bladder. The transverse process of the fourth vertebra has a broad 

 flat root, and may be simple, or, as is more usually the case, cleft 

 more or less deeply into anterior and posterior divisions, of which the 

 former is always decurved for a portion of its extent, and closely 

 applied, even if not attached, to the lateral portion of the anterior 

 wall of the bladder, in addition to its ligamentous or articular con- 

 nexion with the post-temporal. In certain Siluroids the transverse 

 process becomes modified to form the " elastic-spring-apparatus," first 

 described by Johannes Muller. In some of these forms (Malapterurus, 

 Synodontis, Pangasius) each anterior division is almost completely 

 separated from the posterior division, with an oblique origin from 

 the arch of the complex vertebra, and, becoming flexible and 

 highly elastic, expands distally into a more or less oval plate, which 

 is closely applied to the lateral portion of the anterior wall of the 

 bladder. In others (Auchenipterus, Oxydoras) the anterior division 

 also forms an " elastic-spring-apparatus," but the posterior division is 

 entirely wanting. In all such cases the modified transverse processes 

 are provided with powerful protractor muscles, which have their 

 origin on the posterior face of the skull, and their insertion into the 

 anterior surfaces of the oval plates. 



The post- temporal bone alwajs has a transversely, or obliquely, 

 disposed inferior limb for articulation at its inner extremity with the 

 lateral surface of the basioccipital, in addition to an ascending process 

 for articulation with the pterotic and epiotic bones. Where the trans- 

 verse process of the fourth vertebra fails to articulate with and 

 support the post-temporal, as is the case in all Siluroids possessing an 

 " elastic-spring-apparatus," the inferior limb of the latter is exception- 

 ally massive, with an extensive articulation, or even partial anchy- 

 losis, with the basioccipital or, in addition, with the exoccipital also. 

 In other genera (Macrones, Bagrus, &c.) the inferior limb, in conjunc- 

 tion with the body of the same bone, may form a bony expansion or 

 post-temporal plate, which, with the produced crescentic distal 

 extremity of the anterior division of the transverse process of the 

 fourth vertebra, forms a slightly concave bony structure for the 

 support of the lateral portion of the anterior wall of the bladder. 

 From being but faintly concave on its posterior face, the post-temporal 

 plate and the adjacent portion of the inferior limb may become deeply 

 excavated to form a goblet-shaped cavity, into which a thin- walled 

 ccecal diverticulum of the air-bladder extends (Macrones aor). 



Apart from those which are characteristic of all Siluroids, no 

 important modifications of the hinder part of the skull are observable 

 in the normal members of the group, either as regards the more 



