2 Mr. A. AV. Waters on 



internally at the aperture. In a limited area, like tiie 

 Mediterranean, measurements give the readiest means of 

 recognising Cyclostomatous species, but in comparisons of 

 species from distant localities we must not expect as much 

 uniformity. Zorocial and zoarial measurements arc usually 

 good characters, but may be one of the first to change. 



Introduction. 



Idmonea, Lamouroux, has received much study, but nearly 

 all that had been written, until the ovicells became available 

 as a character, may be considered valueless, and the same 

 is the case with a great part of the Cyclostomata. 



We are now agreed that the diti'erent forms of ovicell are 

 very important, and much more attention is being given to 

 the ovicells and their ducts. Canu and Bassler'^, in a large 

 and most important work, give particulars of a large number 

 of ovicells, and base their classification largely on the ooecio- 

 stome, and my present investigations are, to a large extent, 

 directed to seeing to what extent we can rely upon the 

 characters used. 1 have referred to the importance of the 

 ovicell in many papers, and to me Messrs. Canu and Bassler's 

 work is most interesting in giving particulars in such a large 

 number of cases. 



However, do we yet know how much value must be given 

 to each character? And there are cases where I should have 

 considered that the ovicells gave only specific (and not generic 

 or even family) characters, and this has now to be studied. 

 For example, the ovicells in Diastopora are most useful, and 

 we can thereby distinguish, for example, D. obelia, J., and 

 D. patina, L.; but are the diflferences, which are mainly in 

 sha[)e, sufficient for generic (much less family) divisions ? 

 The type is the same. Other characters may support or 

 disprove present views. Further, the ovicell of Plagioecia 

 and Microd'cia^ Canut, is very similar. Canu places D. surni- 

 ensis, Norm., under Microa'cia as the type, but the Guernsey 

 type-specimen in the British Museum has wide tangential 

 ovicells, including many zooccia : the oocciostome is slightly 

 funnel-shaped with a wide funnel (0'08 mm.), also my 

 Guernsey specimen, determined by Norman, has fairly large 

 tangential ovicells, though broken down. Hincks has also 

 described D. sarniensis with ovicells transversely elongate 



• "N. Amer. Earlv Tert. Rrv.," U.S. Natiotial xMus. JUill. 100 {\[}2(i). 

 t Loc. p/A pp. .326-327. 



