118 IOWA GEOLOGICAL SURVEY 



four in number, externally covered by a membranous fold, so that 

 but one opening appears on the outside ; pelvic fin of male with 

 clasper. 



"We may profitably insert here the following paragraph from 

 Smith Woodward relating to the Holocephali, and also a few 

 general remarks from the same source regarding Chimaeroids, 

 living and fossil. For further details one may consult the elab- 

 orate monograph on Chimaeroids by Professor Bashford Dean, 

 published by the Carnegie Institution of Washington, 1906. 



Dental plates essentially similar to those of the existing Chim- 

 aeroid fishes are met with in rocks as early as the Middle De- 

 vonic ; but there is still no evidence of any member of the Holo- 

 cephali which can not be included in the surviving order of Chim- 

 aeroidei. Some of the early forms were certainly armed with 

 dermal plates; but palaeontology as yet lends no support for 

 the theory that the Chimaeridae are degenerate descendants of 

 fishes once possessed of membrane bones. The earliest known 

 complete skeletons are unfortunately only Liassic. 



Order CHIMAEROIDEI. 



In all known Chimaeroids, whether recent or extinct, the noto- 

 chord is persistent and at most only partially constricted, the 

 calcifications in the sheath, when present, consisting of slender 

 rings more numerous than the neural and haemal arches The 

 pectoral fins are abbreviate, without segmented axis; and the 

 pelvic fins in the male are produced into a pair of claspers. In 

 the extinct forms there is no trace of any dermal plate developed 

 in the opercular flap. 



The only clear evidence of evolution hitherto observed con- 

 cerns the development of the peculiar dental plates. In each of 

 the four known families the dentition consists of a few large 

 plates of vascular dentine of which certain areas ("tritors") 

 are specially hardened by the deposition of salts within and 

 around groups of medullary canals, which arise at right angles 

 to the functional surface. In most cases there is a single pair 

 of such plates in the lower jaw, meeting at the symphysis, while 

 two pairs (the so-called vomerine and palatine plates) are ar- 

 ranged to oppose these above. A permanent pulp remains under 

 each plate, and growth thus takes place continually within as the 



