84 ILUNOIS BIOLOGICAL MONOGRAPHS [84 



This author has stated that in some of the forms with a thick cutis, the lateral 

 line canal ossification never fuses with the squamosal element and hence both 

 are independent ossifications. The internal relations of the pterotic lamella 

 are the same as in Amiurus, and cartilage persists to a great extent within it and 

 the surrounding bones. In those forms where the sphenotic is small (Scomber, 

 Salmo, the Loricati, etc.) and limited to the anterior edge of the otic capsule, 

 the squamosal element of the bone extends anteriorly and articulates with the 

 frontal. The lateral line canal, which in Amiurus passes first through the 

 sphenotic and thence into the squamosal, in these forms passes directly from the 

 frontal into the squamosal. The squamosal element of the bone is usually 

 limited medially by the parietal, but in Amiurus it has fused with the supra- 

 occipital. There is always a ridge or a groove on the ventral surface of the 

 squamosal part for the support of the shoulder girdle, and another groove on 

 the lateral face, for articulation with the hyomandibula. 



Cuvier called this bone the *os mastoideum,' homologizing it with the mas- 

 toid portion of the temporal bone of human anatomy. Halhnan (1837) recog- 

 nized it as the homologue of the squamosal element of the temporal, and, re- 

 garding the bone from adult conditions, this was a logical conclusion, as he did 

 not study its development. Both of these authors used the cranium of Perca 

 and Hallman also made a detailed study of the cranium of Cyprinus. Huxley 

 (Esox) called the bone the squamosal and did not recognize its relation to the 

 chondrocraniimi. Parker (1872) says that he tried to point out to Huxley the 

 fact that in Salmo the bone developed in connexion with the wall of the lateral 

 semicircular canal and hence was comparable to another otic ossification cen- 

 ter. Huxley would not entirely grant his views, but in his book remarked that 

 in the opinion of Parker, the bone under consideration was a true otic bone. 

 Thus it remained for Parker (1872) to designate it as the pterotic in Salmo. 

 According to this view it was developed entirely as a chondrocranial bone and 

 did not have the elements which were recognized later by Gaupp and Schleip. 

 They called it the dermo- and autosquamosal according to the suggestion of 

 Van Wijhe (1882) for the naming of the parts of mixed bones. Sagemehl found 

 both elements in the Characinidae and the Cyprinidae, but called the whole 

 the squamosal. Allis calls this bone by differing names in his papers, and in 

 his work on the Loricati (1910) designates the bone as the pterotic followed by 

 squamosal in parenthesis, giving an erroneous synonomy to the first term. 



The epiotics. These are a pair of bones on the latero-posterior dorsal angles 

 each of the cranium. Each lies between the squamoso-pterotic, the supraoc- 

 cipital and the exoccipital of its side (Figs. 9, 10). Each is pyramidal with the 

 apex on the posterior surface of the cranium and has three faces, the first on the 

 dorsal surface of the cranium, the second on the posterior surface below the 

 apex, and the third on the la tero- ventral surface (Fig. 6). The dor so-anterior 

 surface of the bone bears a strong vertical crest, along the anterior face of which 

 some opercular-mandibular muscle fibres have their insertion. The median 



