786 EEPORT OF COMMISSIONEK OF FISH AND FISHERIES. [40 | 



The entire organization has become changed. A pleasing, graceful 

 structure has taken the place of the clumsy Selachian skull. The deli- 

 cate and rounded contours of the latter are replaced bj angular, and 

 quite often by oddly-shaped skulls, on which the grooves for muscular 

 attachment and tendon insertion are distinctly marked. The new ma- 

 terial substituted for the building up of these structures far surpasses 

 the old, not only in its capacity for resistance, but also is greatly supe- 

 rior to it in its fitness for plastic modelling. In this particular, one finds 

 very marked gradations even among the higher fishes. In their rounded 

 contours, and in the imperfectly developed muscular grooves and crests, 

 the bony Ganoids and a number of the Physostoma remind one very 

 much of the Selachians ; and it is only in those groups of fishes exhibit- 

 ing the highest development, more particularly Acanthopterygii, that 

 the types of extreme differentiation come into bold relief. 



Leaving out of consideration the fact that it partly consists of differ- 

 ent material, the primoidal cranium shows but few points of difference 

 from that of the Selachii. In the first place, by the co-ossification of 

 several vertebrae, the occipital region in Amia has attained a distinct 

 morphological value, differentiating it from the corresponding regions 

 in the Selachians, without having its form essentially changed by the 

 process. Compared with the Selachians it has increased considerably, 

 but in length only, which is sufiiciently accounted for by the circum- 

 stance just mentioned. 



The posterior part of the skull cover, in the vicinity of the occipital 

 region, presents a structure that already esssentially exists in the Se- 

 lachii. The median, cartilaginous process, pointing posterially, is 

 present in the Notidanides, being developed there as a cartilaginous 

 crest. ]^or is it difficult to recognize in the medial projections occupied 

 by the exoccipitals in Amia, the cartilaginous elevations developed upon 

 the projecting posterior arches of the Selachians. The posterior lateral 

 angles of the skull, formed in Amia by the intercalare, are also very 

 well developed already in some of the sharks, as, for example, in Scyl- 

 litim. Between the crest of the posterior arch and the last-mentioned 

 lateral projection of the skull in the Scyllia there can already be recog- 

 nized a depression in the cranical vault, extending into the region of 

 the postorbital process, which in Amia is bridged over by the overlying 

 dermal bones, closing in the temporal fossie. In the region of the laby- 

 rinth of the Selachians we find this cavity closed ui> on the side towards 

 the cavum cranii ; in Amia it is widely opened, probably a fenestration 

 proceeding from the perijihery of the acusticus foramen. 



Upon the outer aspect of the labyrinth region, the changes occasioned 

 by the presence of the articular facet for the hyomandibular, are the 

 most striking. I have already availed myself of the opijortunity to 

 point out, in the higher fishes, the extension of the hyomandibular for- 

 wards as far as the postorbital process. 



