83] THE SKULL OF AMI URUS— KINDRED 83 



articulates with the sphenotic, medially with the supraoccipital, and posteriorly 

 with the epiotic (Fig. 10). 



There is no temporal fossa, but there is a space between the squamosal and 

 pterotic parts, which may be a remnant of it. The latero-posterior end of the 

 bone projects broadly and on its surface is the groove for the lateral line canal 

 before it enters the post-temporal. Connective tissue fibres extend from its 

 margin to the margin of the operculum, holding the latter in place. 



The ventral surface of the bone is grooved and ridged for the insertion of 

 the fibres and ligaments of the adductor operculi muscle (Fig. 6). This face 

 of the bone is sculptured more than is the dorsal; it is formed by the ossification 

 of the fascia and connective tissue external to the original perichondria! ossifi- 

 cation. The latter ossification shows in two places on this surface — anteriorly 

 on the margin below the hyomandibular articular surface, and posteriorly on 

 the postero-ventral margin of the bone. In both of these places the bone is a 

 very thin lamella separated from the bone nearest it by cartilage ; the cartilage 

 is continuous within the lamella of both of the bones. Anteriorly it is separated 

 from the sphenotic and posteriorly from the exoccipital by cartilage. Between 

 these two bones the outer lamella interdigitates with a similar lamella of the 

 prootic, and close examination of this part shows considerable cartilage yet 

 remaining beneath these layers. Postero-dorsally in interdigitates with the 

 epiotic. 



The dorso-lateral margin of the bone is grooved for articulation with the 

 hyomandibular and is entirely ossified in all parts. As noted above, this facet 

 for the hyomandibular continues anteriorly on to the sphenotic. There are 

 several foramina in the ventral surface of the bone which lead into its interior, 

 but do not communicate in any way with the cavum cranii. Several blood ves- 

 sels and some loose connective tissue fill this space and the region immediately 

 external to it. 



Internally, the pterotic lamella is limited to the lining of the recess for the 

 lateral semicircular canal and the anterior wall of the posterior (Fig. 7). In 

 the floor of the recess it interdigitates with the prootic lamella, but is separated 

 from the internal lamellae of the other bones — supraoccipital and epiotic — by 

 the cartilage of the lateral septum and the roof of the recess. In the very lateral 

 extremity of the recess, the cartilage has been resorbed, a process beginning in 

 the 32 mm. stage (Fig. 31). For the most part, however, irmer and outer peri- 

 chondria! lamellae are separated by persisting cartilage. 



The squamosal part of the bone is the only part found in Amia (Sagemehl, 

 1884), and it is distinctly separated from the chondrocranium by a space filled 

 with connective tissue and the anterior ends of the body musculature. There 

 is also a lateral line element fusing with the squamosal in Amia just as in Ami- 

 urus, and the canal, upon leaving the squamosal, pursues the same course. 

 Amiurus repeats the condition which Sagemehl described for most of the Chara- 

 cinidae and Cyprinidae, where a squamosal unites with a pterotic element. 



