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PROFESSORS T. W. BRIDGE AND A. C. HADDON 
ossifications. In all forms in which these processes are present they form a rigid 
support to the mesial portion of the anterior wall of the air bladder, and are usually 
cleft (Anchenoglanis) or perforated at the base [Arius) for the transmission of the 
dorsal aorta. 
In the pectoral girdle the post-temporal bone always 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, and a descending process or stem for articulation with 
the clavicle. The socket for the clavicle is usually formed by a deep cleft in the 
stem, which is often more or less completely closed by the opposition of the trans- 
verse process of the fourth vertebra. Where the transverse pi'ocess 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 exceptionally massive, with an extensive articulation, or even partial anchylosis, 
with the basioccipital, or, in addition, with the exoccipital also. In other genera 
{Macrones, Bagriis, dec.) the inferior limb, in conjunction 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 cmcal diverticulum of the air-bladder extends [Macrones aor.). 
Where post-temporal plates are formed the sockets for the clavicles are in the form 
of tubular canals traversing the substance of the plates. 
Apart from those which are cliaracteristic of all Siluroids, no important modifi- 
cations of the hinder part of the skull are observable in the normal members of 
the group, either as regards the more general features of structure, or the more 
special points involved in the mode of formation and relations of the “ cavum sinus 
imparis,” or of its bilobed backward prolongation, the “ atria sinus imparis.” The 
uniformity in the latter respect is so marked, that a description of those structures 
as they occur in any one normal Siluroid {c.g., Macrones) will practically apply 
to all the others. 
Of the minor variations we may remark the occasional translucency of certain of 
the periotic bones, such as the prootic and epiotic bones, and the opisthotic plates of 
the exo ccipitals ; the strengthening of the posterior wall of the atrial cavities by a 
column-like process of bone derived from the dorsal margin of tlie posterior face of 
the basioccipital, or by a slight downward growth of the posterior margin of the roof 
of the cavum sinus imparis, and the withdrawal of the external atrial apertures 
altogether within the neural canal {e.g., Aspredo). 
As regards the internal ear, the condition of many of our specimens was such 
