PEOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 411 



or less along tlie iiortbeasteru coast of Kew England, and northward, 

 two other tolerably common chitons, one of them Tracliydermon albus 

 Linne, which does not go to great depths, as far as known, either in the 

 Atlantic or Pacific. In Alaska it is abundant from low-water to 100 

 fathoms. The other, LeptochUon cancelJatus Sowerby, occurs off the 

 British Possessions, and may reach a depth of 300 fathoms. Earer 

 species, which may be found in deep water, are Leptochiton alveolus 

 Sars (150 fathoms Gulf of Maine); Hanleyia mcndicaria Mighels and 

 Adams ; H. dehiJis Gray (to 300 fathoms) ; and II. tropicalis Dall, from 

 southern waters (Sand Key, 128 fathoms). 



The greatest depth from which chitons have been reported is 1,00G 

 fathoms, at which the Leptochiton Belknapi Dall, was obtained in the 

 IsTorth Pacific. It has since turned up from Kerguelen in the Challen- 

 ger collections, and, perhaps, may eventually' be found in the North 

 Atlantic. 



NOTES ON THE GENERA. 



The slender side teeth of Lepeta are distinguished from true laterals 

 by not being situated on the central longitudinal area of the radula. 

 By their form alone it would be impossible to distinguish them from 

 teeth which are truly laterals, like the inner laterals of LepcteUa. 



Since 18G0 (when I revised the classification of the Lcpetid(Vi\\\i\^ some- 

 what later, of the order to which they belong), little by little informa- 

 tion has been coming in which tills the gaps then known to exist in our 

 knowledge of the order. It is now possible to review more understand- 

 ingly the relations of the dentition of the different groups. It would 

 seem at first sight as if the dentition of Lepeta and Lepetella differed 

 very widely, but more reflection diminishes the apparent divergencies. 



It may be suggested that in Lepeta cwca. the large rhachidian tooth 

 really represents a consolidation of the six laterals characteristic of 

 Acmwa, which is supported by the fact that G. O. Sars* figures the lat- 

 eral cusps of the rhachidian tooth in niidium fulvum as accessory rather 

 than inherent parts of that tooth, a view (I find on rel'erence to them) 

 supported in part by my own original drawings, and a condition which, 

 though not universal nor necessary, may yet be characteristic of some 

 stages of the development of the individual or of the radula ; or perhaps 

 of some individuals merely, while in others the consolidation goes so 

 far that the sutures (as in the bony structures of higher animals) are 

 obliterated. In that case the rhachidian tooth of Ijepetella would rep- 

 resent the consolidation of the two inner laterals merely, if the number 

 six be taken as typical, which, from its universality elsewhere in the 

 order, 'we may reasonably assume to be the case. This is the typical 

 number in the Taniioglossa to which (as Professor Verrill indicates in 

 his description) the radula of Ijepetella is in some respects aimlogous; 

 though the Twnioglossa have no uncini. In the same way, as has before 



* MoU. Reg. Arct. Norveg. tab. II, fig. 12. 



