28 ILLINOIS BIOLOGICAL MONOGRAPHS [28 



dorsally with the wall of the otic capsule and the other descending posterior 

 to, but separate from, the quadrate cartilage. Unhke Amia, Amiurus and 

 Salmo, it does not enclose the ramus hyomandibularis facialis. In Amia 

 (Van Wijhe, 1882) the palatine and pterygoid cartilages are fused and have an 

 articulation with the ventral surface of the ectethmoid process. In the latter 

 forms the hyomandibular has a greater extent of articulation than in Polypter- 

 us, but the foramen for the hyomandibularis nerve is nearer the anterior edge 

 of the cartilage in Amiurus than it is in Salmo. In the latter the quadrate 

 lies much farther anterior to the bulk of the hyomandibula than it does in 

 Amiurus, and a slender cartilaginous process descends below it from the 

 anterior end of the hyomandibula. This is the symplectic element which is 

 not independent in Amiurus, but is fused with the quadrate-hyomandibular 

 mass of cartilage and may be the part which descends behind the quadrate 

 and connects with the interhyal. 



In all of these forms Meckel's cartilage has the same general shape and 

 relations. The articular surface may be convex as in Amiurus or concave as 

 in Acanthias. Usually a small bit of the posterior end projects beyond the 

 quadrate. The coronoid process on the dorsal surface of the cartilage is well 

 marked in Poh'pterus, but does not project as abruptly as in Amiurus. In 

 Salmo the cartilage has practically the same size from anterior to posterior ends. 



THE SKULL OF THE 32 MM. LARVA 



The description given in the following section is based upon the study of 

 the head region of 20 mm., 32 mm., and 60 mm. larvae. The first two stages 

 were specimens of Amiurus nebulosus (catus), and the third of A. melas. A 

 wax model of the cranium of the 32 mm. stage was made, as it gave all of the 

 typical perichondria! and dermal ossifications at an intermediate stage in their 

 development. One side of the model was left without osseous parts, to facili- 

 tate comparisons, as is commonly done in modelling of this kind. The 20 and 

 60 mm. stages were used to supplement this. 



Nearly the whole roof of the cranium at this stage is covered by either 

 perichondria! or dermal ossifications (Fig. 3). The former are derived from 

 the ossification of the perichondrium of the chondrocranium and the latter 

 from the ossification of connective tissue membrane external to it. Sometimes 

 the two elements are intimately fused. Another type of bone development 

 may be mentioned here, the ossification around a lateral line canal. The 

 development of this type of bone has been worked out in detail by Piatt (1893) 

 and Klaatsch (1895). 



The roof is no longer widely open as in the younger stage, but the fontanelles 

 are limited to narrow slits, anteriorly between the frontals and posteriorly be- 

 tween the parietal part of the supraoccipital ossification (Fig. 3). The nasal 

 region has an internasal septum which has grown up from the floor of the 

 ethmoid plate, separating the olfactory lobes (Fig. 22). 



