The Anatomy of Chlamydoselachus 493 



in some features it seems more primitive than any other living shark; second, in certain 

 other respects it is highly specialised; third, it possesses some characters that are unique; 

 fourth, it combines (as in the spiral intestine) some characters that are ordinarily segrc 

 gated in different species; and fifth, it is highly variable. Within obvious limits, the 

 frilled shark is a comprehensive type, and this constitutes one of the difficulties in the 

 way of determining its affinities. 



It is recognised that we are here on treacherous ground. Opinions will differ con' 

 cerning the evaluation of the anatomical characters of Chlamydoselachus, and concerning 

 the status of the animal as a whole. Nevertheless, to give point to the discussion I have 

 summarised the most important data (Tables IV and V, pp. 496-497) in two lists of 

 characters: one palingenetic or primitive, the other cenogenetic or of relatively recent 

 origin, with reference to comparable structures in other living sharks. Some very obvious 

 features, such as the unusual number of gilPsHts and the dorsoventral flattening of the 

 head, are excluded because of insufficient evidence as to their status. It is not expected 

 that anyone will accept either list in its entirety. Each list might be greatly extended, 

 affording endless opportunities for debate. 



The more striking peculiarities of Chlamydoselachus, such as the very elongate 

 form of the body and the peculiar hyostylism of the skull, are obviously cenogenetic. 

 The real difficulty lies in the disguises which may conceal other cenogenetic characters. 

 Apparent primitiveness is frequently the result of degeneration or retrogession, in 

 a phylogenetic sense; this, as applied to the individual, is usually a matter of arrested 

 development. In Chlamydoselachus there are evidences of retrogression in the skeletons 

 of the fins, in the mesonephric duct and urinary sinus of the right side, and in the vestigial 

 seventh gill'arch. In each case there are decided irregularities. It seems to be a fairly 

 general rule that, when the development of an organ is arrested, it does not merely fail 

 to attain the ancestral condition, but exhibits a vestigial complexity. 



In Chlamydoselachus there are features, such as the thin walls and large foramina 

 of the cranium, the incipient cyclospondylous vertebral centra, and the paired condition 

 of the urinary sinuses in the adult, that appear more characteristic of an immature than of 

 an adult shark. The position of the epibranchial arteries is that found in the embryos 

 of other sharks. In all these cases there is no evidence that development has ever gone 

 further. The alternative is to accept these features as primitive characters. The per' 

 sistent thyroglossal duct may be anomalous, since it is not found in all specimens. Since 

 the so'called duct differentiates like the wall of the pharynx, from which it is derived, 

 it is obviously something more than an embryonic rudiment. 



I have said that, within obvious limits, Chlamydoselachus is a comprehensive type. 

 This is true mainly with respect to features that may be found in other sharks, but some 

 of the resemblances to higher vertebrates are striking. Of these, it is sufficient to mention 

 the extreme length and mobility of the jaws, suggestive of the Ophidia; the gular fold, 

 simulating a condition found in many of the Teleostomi; and the armature of scales on 



