82 ILLINOIS BIOLOGICAL MONOGRAPHS [82 



Cyprinidae (Sagemehl, 1891), and in Salmo (Parker, 1872). It is not remarkable 

 to find it where the chondrocranium is persistent to a very great degree in other 

 parts, but in Amiurus, where there is a great deal of ossification, it is significant 

 of the primitiveness of this region. 



In some of the Clupeoid fishes (Ridewood, 1904), the basicapsular fenestra, 

 which occurs in larval Salmo (Parker, 1872; Gaupp, 1906), is a constant feature 

 in the adult between the prootic, parasphenoid and basioccipital bones. 



In Perca (Cuvier, 1826; Hallman, 1837), Carpio (Cuvier, Hallman, Stan- 

 nius, 1853; Sagemehl, 1891), Pleuronectes (Cole and Johnson, 1901), the prootic 

 forms the lower part of the hyomandibular facet. In Salmo, Scomber, the 

 Loricati, and m.any other forms, there are separate foramina in the bone 

 for the passage of carotid arteries, jugular vein and the hyomandibularis ramus 

 of the facialis. In Amiurus, some, but not all of the blood vessels communicate 

 with the internal parts of the cranium through the large trigemino-facial fora- 

 men. In Scomber, the external surface is grooved as in Amiurus for the inser- 

 tion of the adductor hyomandibularis; the same thing is possibly true for most 

 £shes, but has not been stated in the descriptions. 



The sqimmoso-pterotlcs. In the 32 mm. stage both the squamosal and the 

 pterotic were in their initial stages and the perichondrial and dermal elements 

 which make up the adult compound bone were just beginning to fuse in the wall 

 of the otic capsule above the lateral semicircular canal (Fig. 31). In places, the 

 cartilage had been resorbed and the inner and outer lamellae were connected 

 by osseous trabeculae. The lateral line canal, which in the older stage, is 

 invisible from above, at that time formed a slight ridge on the dorsal surface of 

 the cartilage and was fused to the underlying perichondrial ossification. In 

 the adult, the outer surfaces of the bone, both lateral and dorsal, are made up 

 almost entirely of ossified membrane and muscle fascia (Figs. 6, 10, 20). This 

 led some authors who had not carefully studied the development, to conclude 

 that the whole bone was entirely a dermal ossification, and hence comparable 

 to the squamosal part of the temporalis of man. 



The lateral line canal ossification of the adult becomes an integral part of 

 the dorsal surface of the bone and connects postero-laterally with the superior 

 end of the opercular-mandibular canal and the main lateral canal of the body 

 (Fig. 11). The former passes into the latero-posterior comer of the bone from 

 the dorsal one of two subtemporal lateral line ossicles, the margin of which is 

 fastened to the squamosal part of the bone by ligamentous tissue. Before 

 leaving the squamosal that part of the canal which is to pass into the body 

 issues to the surface, and runs posteriorly on it for a short distance before pass- 

 ing into the post-temporal. There are two sense organs in the lateral line canal 

 within the squamoso-pterotic. 



The dorsal surface of the squamosal part of the bone is subtriangular in 

 outline and with serrated edges overlaps the surrounding bones. Anteriorly it 



