CHIMMROIDS. 461 



have unquestionably many structures which may have been derived from the 

 more generalised conditions of the sharks ; and as a group they may not un- 

 reasonably be looked upon as descended from the primitive elasmobranchian 

 stem." 



SUB-CLASS II.— HOLOCEPHALI. 



Chiivleroids. 



The group typically represented by the spook-fish or chimrera is, as already 

 mentioned, now a small and waning one, which agrees with the last in the 

 structure of the skull. In spite of their many marked resemblances to the 

 sharks, the writer last quoted is of opinion that they cannot be closely associ- 

 ated with that group. This external resemblance to the Blasmobranchii, 

 coupled with the total absence of membrane-bones from the cartilaginous 

 skull, at once serves to differentiate the chimeeras from the lung-fishes; from 

 which they also differ by the absence of true posterior nostrils, although there 

 are nasal passages. Further differences are to be found in the form and 

 'structure of the paired fins, and the absence of an air-bladder. The optic 

 nerves and intestine are similar to those of the lung-fishes and sharks ; and 

 the accessory reproductive organs of the males, known as claspers, are found 

 elsewhere only in the latter group. Unlike the lung-fishes, the skin is gener- 

 ally naked, and when (as in the young) hard structures are developed therein, 

 these agree with teeth, whereas such plates as are found in the lung-fishes 

 are bony. Although the paired fins are not very unlike those of modern 

 sharks, consisting of an abbreviated base and numerous rays arranged in a 

 fan-like manner, their dermal margins are very large, and terminate in a 

 sharp point. A large spine 

 protects the front of the first 

 dorsal fin, which is capable 

 of being folded so as to lie 

 in a groove in the back. 

 The gill-arches are closely 

 packed, and open by one 

 aperture on each side, which 

 is slit-like, and protected 

 with a dermal fold contain- 

 ing an operculum of carti- 

 lage. The teeth are in the 

 form of large plates chisely Fig. 3.— Common Cuim^ka. 



affixed to the jaws, and ai'e 



thickly studded with roughened grinding facets termed tritors. In the car- 

 tilaginous skeleton the axis of the backbone remains notochordal, its sheath 

 lacking distinct vertebra;, but being strengthened in the fore-part of the 

 column by rings of cartilage. A lateral line is present on the sides of the 

 body, and the tail gradually tapers away, to terminate m a diphycercal fin. 

 Like sharks, chimjeroids deposit large eggs, which are completely separate 

 from one another. , . , 



Existing chimKroids are classed under three genera, all included in the 

 family Chimceridx. The first of these is Harriotta, which comprises two deep 



