[61] THE OSTEOLOGY OF AMIA CAEVA. 807 



Some of these cranial bones may be considered to form certain groups, 

 of which four enter into the occipital region. These are the basioccipital, 

 the two exoccipitals, and the snpraoccipital. In Micropterus, as in all 

 fishes, the basioccipital is the direct continuation of the spinal column, 

 and i^ossesses many of the characters pertaining to the vertebrae. The 

 exoccipitals inclose the aperture of the foramen magnum entirely in 

 this fish, only partly in many others. The snpraoccipital, or the upper 

 segment, supports a median crest that corresponds to the neural spine 

 of the vertebra. Its form varies exceedingly, as well as its size. 

 Another group of bones inclose the ear capsule, and have had names 

 bestowed upon them that denote tJie relative position they bear to it. 

 JFirst and most constant among these is the prootic ; it is either pierced 

 by or limits the foramen for the trigeminal nerve. Second in order 

 <!iome the epiotic, which overlies the superior vertical semicircular canal. 

 It, in the vast majority of fishes, always forms a projecting process. 

 Next we have the opisthotic, a segment lying in front and to the side of 

 the exoccipital, but does not appear on the internal aspect of the brain- 

 case nor bear any direct relation with the labyrinth. To these three 

 bones Mr. Parker added a fourth, the pterotic, which in fishes forms the 

 posteroexternal angle of the cranium. In most bony fishes it articu- 

 lates with the outer limit of the posttemporal. I may add here, in pass- 

 ing, that these bones form the " periotic mass," and are the same found 

 in the petro-mastoid portion of the temporal in man and the other 

 higher vertebrates. It is unnecessary to say more than I already have 

 above about the squamosal and postfrontal. The latter is sometimes 

 termed the sphenotic, and assists often the former in the formation of 

 the articular facet for the hyomandibular. This is the case in Microp- 

 terus. 



Beyond these, in another group, we have the alisphenoid at the sides 

 and behind and the orbitosphenoids anterior to them, while the basi- 

 sphenoid is found below. This latter bone, we have already shown in 

 the black bass, forms an osseous partition between the two sets of or- 

 bital muscles ; it may be absent in some of the Teleostei and very small 

 in others. 



On the cranial vault the parietals are not always separated from each 

 other by the intervention of the supraoccipital as they are in Micropterus^ 

 nor are the frontals always separate bones, they sometimes forming only 

 a single piece, as in Gadus. The segments of the ethmoidal region have 

 been sufficiently described above. They all, the prefrontals, ethmoid, 

 and vomer, vary greatly in size, form, and relationship throughotit the 

 class. 



To still further illustrate the relations that may exist among the bones 

 in crania of osseous fishes, as well as some of the remarkable forms they 

 may assume, I am indebted to Professor Gill for the loan from his private 

 cabinet of the cranium and portion of the palato-quadrate arch of a speci- 

 men of Alhula vulpes and an imperfect cranium of Megalops, the latter 



