H. L. Hawliins — Dental Apparatus of Discoidea. 151 



impossible to jutlge certainly as to the relative ages of the specimens, 

 but the fact that the auricles in my example are not quite so fully 

 developed as in the original of Loven's figure on p. 49 seems to indicate 

 that my specimen is the younger of the two. Assuming that the jaws 

 described by Loven are those of the tall form, then the smaller 

 symphysial proportion in the present specimen lends confirmation to 

 this belief. 



Perhaps the most striking feature of the ' lantern ' of B. cylindrica 

 is its strongly 'regular' character. In man}' respects the structures 

 closely resemble those in Cidaris papillata. The slightly less elevation 

 of the outer faces of the pyramids, the internal keel of the tooth, and 

 the slanting position on the perignathic girdle, are the only obvious 

 differences, save for a slight tendency in the epiphyses of Cidaris to 

 converge to form an arch. The epiphyses of Biscoidea are verj- like 

 those of the Diademine genus Asthenosoma, but in that form again the 

 tooth is grooved on the concave side. 



The great similarity between the dental structures in Biscoidea and 

 the regular Echinoids tends to confirm the belief that the Discoidiidae 

 are ver}^ primitive among irregular Gnathostomata. In the structure 

 of the jaws, at least, these forms might well be regarded as inter- 

 mediate between the Regularia and the Clypeastrina, for the pyramids 

 bear a strong regular facies, Avhilst their method of articulation with 

 the perignathic girdle, and the slight modifications that they do 

 present, all seem to tend towards a typical Clypcastrine character. 



Summary. — The maxillae of a specimen of Biscoidea cylindrica are 

 described, and found to differ somewhat from those described by 

 Loven in " Echinologica". The epiphyses and teeth oi B. cylitidrica 

 are described for the first time, and are shown to resemble those of 

 several Regular Echinoids. 



The differing characters of the perignathic girdle, which were con- 

 sidered by Loven to be of almost specific impoi'tance, are not found to 

 be confined to the different forms of test, and are regarded as differences 

 due to the age of the specimens. 



The intermediate character of the family of the Discoidiidte between 

 Regular and Clypeastrine Echinoids is emphasized. 



In conclusion, I wish to express my thanks to Dr. Hoyle, of the 

 Manchester Museum, for permission to develop and describe the 

 specimen ; also to Dr. Bather for his help and encouragement, and 

 especially for lending me the proof-sheets of his forthcoming memoir. 



LITERATURE REFERRED TO. 



1S74. T.Wright. Monogr. Brit. Foss. Echinodermata. — Cretaceous: I. Eclii- 



noidea; pt. vi, Echiuocouid;Te, pp. 198-213, ]As. xlv-xlvii. (Presence of 



jaws asserted without proof.) 

 1886. P. M. Duncan. " On the Perignathic Girdle of the Echinoidea" : Jouru. 



Linn. Soc. ZooL, vol. xis, pp. 179-212, pis. x.xx, xxxi. (Terminology 



of perignathic girdle.) 



1888. SvEN Loviix. " On a recent form of tlie Echinocouidte " : Bihang Kongl. 



Svenska Vet. Akad. Handl., Bande xiii, Afd. iv. No. x, Stockholm. 

 (Describes one form of girdle in D. cifliniirica.) 



1889. P. M. Duncan & W. P. Sladen. " jSTote on Perignathic Girdle of 



B. cj/luidrica" : Ann. Mag. Nat. Hist., ser. vi, vol. iii. (Expresses 

 disbelief in the existence of jaws in this genus.) 



