ORDER ELASMOBRANCHEI. 939 



garded by Mr Smith-Woodward as but very slightly removed 

 from the original primitive condition ; Cestracion, in which the 

 hyomandibular becomes distinctly differentiated, being a step in 

 advance. In time this genus is definitely known to range 

 from the Middle Jurassic of the Oxford Clay to the present 

 day ; it has indeed been recorded from the Lias, but the deter- 

 mination is more than doubtful. Nearly all the described species 

 are European, but one has been recorded from the Tertiary of 

 New Zealand. Some of the fossil teeth show signs of wear at 

 their summits, and it thus seems that they must have been firmly 

 implanted in the jaws like those of the Hybodonts ; specialisation 

 having apparently tended to produce a loose dental articulation 

 throughout the section. The genus Chlamydoselache has a terminal 

 mouth, with lateral teeth similar in both jaws, and consisting of three 

 slender cones separated by smaller cusps ; the notochord being partly 

 calcified. This genus is now known by a single living species from 

 the Japanese seas, but teeth from the Pliocene of Tuscany have 

 been referred to it. It will be observed from the figure that the 

 mandibular articulation of Notidanus is placed far behind the 

 cranium proper, and it is noteworthy that a similar condition ob- 

 tains in Pleuracanthus among the Ichthyotomi. 



Family CocHLiODONTiDiE. — With this family we enter the second 

 series of the section, in which there are two dorsal fins and five gill- 

 clefts. The present extinct family is an ill-defined one, apparently 

 allied to the Cestraciontidce, but with a more specialised dentition. 

 The dentition is formed from that of the Cestraciontidce (infra) by 

 the welding of at least one of the oblique transverse rows of teeth 

 encircling each ramus of the jaws into a continuous curved plate 

 (fig. 863), which may have either a smooth crown-surface, or may 

 be marked, as in the figured example, by grooves and ridges, indi- 

 cating its compound origin. These dental plates grow by additions 

 to their inner borders, while the outer borders are usually involuted. 

 The dorsal fins were provided with spines. This family comprises 

 a number of genera from the Carboniferous, only some of which can 

 be very briefly noticed in this work. The one most imperfectly 

 known is Helodus, from the English Carboniferous, which appears 

 allied to the next, but does not seem to have had the teeth welded 

 into plates, and therefore differs from the accepted definition of the 

 family. Plenroplax (Pleiirodus) and Psephodus are more typical 

 forms, the latter occurring both in Europe and North America. 

 The dental plates of Psephodus form at least one series of smooth 

 curved teeth, without coronal ridges or involution of the outer 

 border ; and there were also smaller rows of lateral teeth, as well as 

 some prehensile teeth at the extremities of the jaws. Sandalodus is 

 known by the huge dental plates, which are of an elongated trian- 



