338 PEOF. T. W. BRIDGE ON" THE MORPHOLOGY OF THE SKULL 



ai'.c.) is concave from side to side and convex from before backward. It is formed in 

 the centre by the distal extremity of the suspensorial cartilage, strengthened on its 

 inner surface by the hinder part of the palato-pterygoid bone, and externally by the 

 inferior extremity of the squamosal. No trace of a true quadrate ossification could be 

 detected. 



The trabecular region of the chondrocranium may be considered to begin as an 

 extension forward of the cartilage of the auditory capsules in the form of two parallel 

 cartilaginous rods or plates, situated one on each side of the anterior half of the cranial 

 cavity. At its origin from the periotic capsule each trabecular rod is also continuous 

 with the anterior portion of the suspensorial cartilage, and forms a relatively thick plate 

 of cartilage, invested externally by the lateral extension of the fronto-parietal, and 

 separated from the cranial cavity by the ascending lamina of the paraspheuoid, the 

 descending plate of the same bone closely investing its inferior margin (PI. XXIX. 

 fig. 17, tr.c). More anteriorly the rod contracts, and, owing to the diminished lateral 

 growth of the fronto-parietal, now becomes visible in a side view of the skull, 

 externally and parallel to the line of j.unction of the latter bone with the parasphenoid 

 (PI. XXVIII. fig. 1, and PI. XXIX. fig. 16, tr.c.)K More anteriorly still the 

 trabecular rods blend inferiorly with the cartilaginous lamina which, in front of the 

 termination of the parasphenoid, alone forms the basis cranii, and, at the same time 

 increasing somewhat in vertical extent, become overlapped internally by the lateral 

 portions of the fronto-parietal, and partially also externally by the palato-pterygoids 

 (PI. XXIX. fig. 15). On approaching the nasal capsules, each trabecular cartilage 

 detaches itself from the basis cranii, and, still further contracting, assumes the condition 

 of a slender laterally-compressed rod, which curves downward across the outer surface 

 of the fronto-parietal and palato-pterygoid bones (PL XXVIII. fig. 1, and PL XXIX. 

 figs. 13 and 14, an.p.), and then passes ventrally between the antorbital process of 

 the last-mentioned bone and the hinder wall of the nasal capsule. The cartilage now 

 becomes very slender, and, after giving off a short anteriorly-directed process (fig. 1) 

 below and parallel to the outer margin of the nasal capsule, enters the posterior 

 margin of the upper labial fold of its side, and thence describes a bold curve back- 

 ward beneath the eye. Eventually the rod curves downward and slightly forward, 



A similar reduction of the central or interorbital region of the chondrocranium to the quasi-primitive 

 condition of two laterally-situated cartilaginous trabecular rods occurs also in several Urodele Amphibia, such 

 as, for example, Menohranchus (Huxley, 17, and Wiedersheim, 43) and Amjjhiuma (Wiedersheim, I.e.). The 

 resemblance of the latter to Lepidosiren is also heightened by the fact that in both cases the cranial roof-bones 

 suturally articulate on each side with the parasphenoid to form the lateral walls of the cranial cavity. 

 Comparison of transverse sections through the cranium oi Lepidosiren (PL SXIX. figs. 16 and 17) with 

 similar sections of, for example, the skull of Menobrcmclms (Wiedersheim, flgs. 40 and 42) will safficiently 

 iUustrato this point. The occurrence of similar parallel modifications in the Ophidian skull is sufiScient to 

 prove that, however striking may be the structural resemblances between the Amphibian and Dipnoid skulls 

 in these respects, they are nevertheless homoplastic in their natui-e. 



