THE OSTEOLOGY OF AMIURUS CATUS. 287 



and the posterior extremity of the ethino-palatine cartilages. Whether 

 this is really so my specimens do not allow of absolute certainty, but 

 make it a strong probability. Somewhat further back the cartilage 

 is seen and may be traced unbroken back to the quadrate. The 

 metapterygoid of Amiurus combines to a certain extent the relations 

 of the ectopterygoid and entopterygoid, as well as that of the metap- 

 terygoid of other Teleosts, but, since it is in direct relation to the 

 quadrate, and performs the usual function of a buttress to the hyom- 

 andibular, I have preferred the last named term for it. 



The development of the dentary suggests some important thoughts. 

 In a 20 mm. stage, (Fig. 9) Meckel's cartilage Mck) is present in its 

 entirety. On its upper surface is a layer of tooth-bearing bone, in 

 which the individual cement plates (cp) are still to a large extent re- 

 cognizable. At the sides and below is a layer of perichondral bone 

 (pc), the cement plate bone passing into it without any line of demar- 

 cation. In fact both varieties are identical, not only in their histolo- 

 gical features, but also in their origin. Below the cartilage is a 

 mucous canal (MG) enclosed in its osseous tube, which is united with 

 the perichondral bone of the lower surface. In a 38 mm. stage the 

 cartilage has almost disappeared, its place being occupied by trabecu- 

 le of bone, osteoblasts lying in the interspaces. The mucous bone 

 has become quite united with these trabecule, and it is impossible to 

 distinguish it. We have then in the dentary portion of the man- 

 dible what may be termed three different varieties of bone — cement- 

 bone, perichondral-bone (with which may be included the trabecule), 

 and mucous-canal bone. All three, however, pass into each other, 

 and are indistinguishable in structure and origin. The old division 

 into primary and secondary ossification should be done away with 

 since both varieties are in reality similar. 



III.— THE HYOMANDIBULAR, HYOID, AXD OPERCULAR 

 APPARATUS. 



The bones constituting these parts belong to a single arch, the 

 second post-oral, and are in relation to the seventh nerve. 



1. The Htomandibular, (PI. II., Fig. 1, Hmd.) 



Is a large almost quadrate bone, forming the upper part of the 

 arch. It articulates above by a somewhat arched surface with the 

 gphenotic and pterotic, and from the anterior angle of this surface a 



