SEA-URCHINS. 461 



texture tlian the surronndiiig rock. Secondly, the large size of 

 the peristome in tlie earlier genera would be liable to let the 

 jaw-fragments slip tlirough when their supporting muscles had 

 decayed. As all the specnes of Pi/gaster and Ilolectyjnis are nioi-e 

 or less conical in shape, the natural position that the test would 

 assume when allowed to settle on the sea-floor would be with the 

 oral surface downwards. After the jaws had slipped througli 

 the pei'istome they would, on account of their relatively light 

 weight, become scattered by cui'rents which were too gentle to 

 move the whole test. 



Jaws are known to exist in Pygaster, but I have been unable 

 to find desci'iptions or specimens in which their striictuie was 

 adequately shown. From the characters of casts of the pyramids 

 preserved in an ironstone mould of P. ? semisulcatus that I have 

 seen, these parts of the lantern seem to have been large and 

 massive, and of a shape corres[)onding with that of the pyramids 

 of Cidaris. Wright (30) has figured a specimen of llolectypns 

 depressus in which the com})lete lantern is preserved. I have 

 examined the specimen (B.M., E. 1687), but it is impossible to 

 trace any of the ossicles to their extremities, so that no measure- 

 ments of any value can be taken. The general facies of the 

 pyramid is strikingly '• Regular." Nothing seems to be knov.n 

 of the jaws of Anorthojiygns, but they nnist certainly have 

 existed. 



For a hnig time the pi-esence of jaws in IHscoidea was doubted, 

 and sometimes, notably by Duncan (41 & 45), absolutely denied. 

 In 1892, Loven, in the wonderful store of information as to the 

 perignathic structures of Echinoids contained in his Echinologica 

 (Loven, 48), gave a description of the pyramids in D. cylindrica, 

 and recently I was able (Hawkins, 60) to confirm and amplify his 

 description with the additional features of the epiphyses and the 

 teeth. In this genus the pyramids have still a markedly 

 " Regular" appearance, although they were probably much more 

 closely attached to the processes of the pei'ignathic girdle than 

 in any Regular Echinoid. This shortening of the muscles of 

 attachment resulted in a, far less vertical position for the lantern 

 as a whole, while the strong incurving of the adoral parts of the 

 pyramids will have increased the angle to one of about 45 degrees 

 at the peristome. The teeth are cui-ved considerably to correspond 

 with this arrangement. They are strong, and built on the 

 Echinoid plan, in contrast to the Diademoid, with a pronounced 

 keel on the concave side. 



In the case of Comdtts, the long controversy as to the presence 

 or absence of jaws has been partly settled by the discovery of 

 teeth in a specimen of C. sn,brotundus (Hawkins, 65). There is 

 as yet no evidence as to the characters of the jaws ; and the teeth 

 in themselves, beyond their similai-ity to those of Discoldea, show 

 no features of special interest. They are less curved than those 

 of that genus, and more sharply pointed, the latter character 

 being in contrast to what might be expected in view of the 



