54 ILLINOIS BIOLOGICAL MONOGRAPHS [54 



coronoid process corresponding to the autocoronoid of the ganoids (Van Wijhe, 

 1882). The angulare, or possibly the goniale, is represented by a small dermal 

 ossified sheet ventral to the articulare. The lateral ine canal leaves the den- 

 tary at its posterior end and extends ventral to the articulare and thence into 

 the preopercular. In the ganoids the articulare is formed in the same manner 

 as in Amiurus and has been called the autarticulare (Van Wijhe, 1882), but 

 has a dermarticulare attached to it as an independent bone. In Salmo (Gaupp, 

 1906) the articulare is developed from two such elements, the latter containing 

 the lateral hne canal. 



THE ADULT SKULL 



McMurrich (1884) has described the cranium of the adult Amiurus catus, 

 but his description and figures are incomplete and could not be used to supple- 

 ment the points brought out in the first part of this paper. Other authors 

 have made passing reference to the cranium of the Siluroids in general, but none 

 of them give a specific description of the osteology of any one species from the 

 point of view adopted in this paper. Pollard (1895) gives several figures of the 

 chondrocrania of some of the South American forms, with a brief general 

 description of each, but no reference is made to their osteology. Herrick 

 (1901) notices the topographical relations of the cranial bones of Amiurus 

 melas in his discussion of the cranial nerves. 



The cranium of the adult Amiurus nebulosus (catus) is more depressed and 

 flattened than in the later larval stages. The ossifications laid down in the 

 32 mm. stage have invaded and replaced the cartilage in many parts. There is 

 however, more cartilage remaining in the adult cranium than McMurrich 

 (1884b) noted (p. 271): "Very Uttle cartilage remains in the skull, the anterior 

 portion of the ethmoidal cartilage alone remaining imossified. " I find con- 

 siderable cartilage present in the floor of the cranium posterior to the ethmoid 

 region, between the otic capsules (Fig. 7), and in the walls of the semicircular 

 canals. The posterior instead of the anterior part of the ethmoid region 

 remains unossified as the intemasal septum, and will be discussed with the 

 supraethmoid bone. 



The fontanelles of the roof are well described by McMurrich (p. 270). My 

 description of them is more complete, as I have traced their formation from 

 the younger stages and have followed the changes which have resulted in their 

 restriction to the median region of the roof. The ossification which divides 

 them into anterior and posterior fontanelles (Fig. 10), is formed around the 

 epiphysial bar of the larval chondrocranium. Most of the bones of the dorsal 

 surface of the cranium have a dehcate sculpturing. 



The supraethmoid (Figs. 6, 7, 10). This bone in Amiurus has been dis- 

 cribed by McMurrich (1884b), as the 'mesethmoid.' This name implies that 

 the bone is developed within the ethmoid cartilage and not around it, as has 

 been shown for the larval stages. In development, the bone 'arises from der- 



