310 



Bashford Dean Memorial Volume 



Text-figure 28 



Five teeth of Pleuracanthus laevissimus Ag.{ = Di^lodus 



gibbosus) in natural size, to show variations in size, shape 



and secondary cusps. 



After Davis, 1892. 



and si2;e, but are evidently teeth of the same species of shark. Study of them fails to show 

 any generic relationships with Chlamydoselachus. Of these teeth, Davis wrote: 



The teeth of Pleuracanthus laevissimus, Ag., vary greatly in form; there are two prin- 

 cipal cones, circular, or more or less compressed, with or without lateral cutting edges, some- 

 times striated. The cones are divergent, and frequently exhibit a slightly sygmoidal curva- 

 ture. Between the two outer 

 denticles is a smaller intermedi- 

 ate one [sometimes two], which 

 may be short and compressed or 

 comparatively long and slender. 

 On the posterior surface behind 

 the smaller intermediate denticles 

 is a 'button,' which forms the 

 seat on which the anterior part 

 of the base of the succeeding 

 tooth rested. The "button" is 

 of irregular size, sometimes promi- 

 nent, at others scarcely discerni- 

 ble, which is probably due to the different relative positions occupied by the teeth. Base 

 broad, extending backwards, inferior surface more or less flattened. 



Then after describing with some care the varying details of the teeth of several 

 species of Cladodus, all of which have the same general form, Davis concluded with 

 the following summation (1885, pp. 112-113): 



Our knowledge of the fossil forms of Cladodus does not lead us to infer that the teeth 

 of Cladodus differed in form to any extent comparable with those of the existing Chlamydo' 

 selachus. There are no fossil examples known to be associated with the ordinary form of 

 Cladodus similar to those of either the anterior [rows of] recurved teeth or the posterior ones 

 with a single denticle as in the recent fish, and the question naturally arises as to whether the 

 correspondence in the form of one portion of the teeth of Chlamydoselachus is sufficient 

 to justify the assumption that the latter is the existing type of a group of sharks hitherto con- 

 sidered to be extinct since the Carboniferous period. The resemblance is no doubt striking, 

 and the discovery of intermediate forms in the secondary or tertiary formations will be a 

 great assistance in guiding the palaeontologist to a correct estimation of its value, but at the 

 present no such evidence exists and whilst the immense value of the discovery of the fish can- 

 not be too fully appreciated, it may be well to await fresh evidence before finally deciding 

 that Chlamydoselachus is a descendant from the fossil Cladodonts. 



With regard to this matter of a single denticle, it should be understood chat in 

 Chlamydoselachus teeth with a single denticle are comparatively few, since they occur 

 only in the angle of the jaw, where teeth with one or two denticles are sometimes found. 

 Because these teeth are few and presumably not fully developed they should not figure 

 largely in the comparison. 



We have presented the facts concerning the affinities of Chlamydoselachus to fossil 

 forms as they have been set forth by the contenders, freely quoting their very words. 

 To these statements have been added others which we believe to bear on the matter. 



