32 ILUNOIS BIOLOGICAL MONOGRAPHS [32 



posterior in Amiurus than in Salmo. The large tectum cranii is not present 

 in Amiurus. 



The oblique eye muscles of Amiurus have no relation to the ventral surface 

 of the ethmoid cartilage as in Salmo. Because of the development of an 

 interorbital septum, the trabecular wall of the orbit has disappeared and the 

 olfactory nerve crosses the orbit and penetrates the ectethmoid process to 

 reach the olfactory organ. Gaupp (1906) probably selected the salmon as his 

 type of teleost development because of the ease of obtaining material, although 

 it is more highly specialized in many parts in which the Siluroids are almost 

 schematic. The ethmoid region is an example of this, and I think the general 

 development of the ethmoidal region in the Siluroids is more primitive than is 

 the same region of Salmo. 



Gaupp says (06; p. 676): "Die Ersatzknochen occupieren das Chondro- 

 cranium bei den Teleostiem in sehr verschiedenem Umgange; meist bliebt 

 'ein sehr betrachlicher Teil von ihm in knorpeligen Zustande erhalten. Die 

 Zahl der einzelnen Ersatzknochen ist dabei ziemlich gross, aber ihre Aus- 

 dehnung ist beschrankt. Und zwar konnen, wie bei den Ganoiden, nicht zur 

 zwischen den einzelnen Stiicken grosseren Knorpelzonen bestehen bleiben, 

 sondem bei manchen Formen dringen die Knochen auch nur wenig in die Tiefe 

 des Knorpels ein, so dass die Zerlegung des Knorpelschadels in knocherne 

 Territorien sehr unvollstandig sein kann (Alepocephalus rostratus, Gegen- 

 baur). Die Ethmoidalgegend bleibt haufig in grossten Ausdehnung knorpelig. " 



The ectethmoids of Salmo (pleurethmoidale, Gaupp) are formed by peri- 

 chondrial ossifications around the ectethmoid process and a ligamentous con- 

 nexion with the palatine ossifies with them. A large laterally lying dermal 

 ossification such as is found in Amiurus is not mentioned. 



When ossification first appears in Gasterosteus (Swinnerton, 1902), the 

 ethmoid region of the cranium is greatly elongated in a manner somewhat 

 comparable to the condition in Amiurus. The preethmoid cornua are very 

 elongate and have a separate ossification not found in Amiurus, but which 

 Swinnerton compares to the septo-maxillary bones of Amia. Concerning the 

 mesethmoid (supraethmoid, author), he says: "In the anterior portions of the 

 skull the expanded plate-hke portions of the parethmoid cornua have given 

 rise to the parethmoid bones (figs. 4,9, epb), whilst a center of ossification, the 

 mesethmoid, has appeared on the dorsal surface of the corresponding cartilage. 

 The edges of the latter ossification extend freely into the surrounding tissue, and 

 give the impression of a membrane bone whose central portion has united with 

 the cartilage leaving the edges quite free. " 



Evidently Swinnerton did not study the histological relations of this dermal 

 plate to the underlying cartilage or he would probably have observed a condi- 

 tion similar to that in Amiurus and what Allis (1910) found in the young Scor- 

 paena. The latter author says: "The mesethmoid of Scorpaena, although un- 

 doubtedly a so-called primary bone, consists of two distinctly different portions. 



