134 CEYLON PEARL OYSTER REPORT. 



diagram). It is wider than that part immediately in front of the uterine opening, 

 its walls are thinner, and the lining epithelium is flattened and non-ciliated, whilst 

 in the anterior part the lining epithelium is columnar or square and appears to be 

 ciliated. At its hinder end the accessory part of the vagina opens into the accessory 

 vesicle (acc.ves.) which has no muscular Avails; its lining epithelium consists of large 

 square secretory cells, and in its lumen lies a lump of secreted substance. 



The shell glands are very large, they pass their secretion into the anterior part of 

 the vagina (sh.g/.). A very curious feature is the presence of a considerable fold of 

 the body- wall (pi.) on either side of the female aperture. This is easily visible in, 

 the whole specimen when examined with a simple lens, and in conjunction with the 

 still more prominent external male organs serve to distinguish this species at a 

 glance from any other with which I am acquainted. The position of the ovaries is 

 dorsal. 



This is certainly the most interesting species in the collection. It is the type of a 

 genus which may be regarded as a specialized form of Discocelis which has retained a 

 single primitive feature which the latter has lost, namely, the widely separated 

 genital apertures. 



r l he feature most worthy of remark is the development of muscular projections of 

 the walls of the antrum to carry the prostatic glands, which are sessile in Discocelis. 

 They are of importance, since they suggest a probable explanation of the manner in 

 which the curious intromittent prostate organ of the Diposthiidee may have arisen, 

 and because they further give an indication of the manner in which a re-duplication 

 of the penial organs may have come about. In fact, if the genus Discocelis were not 

 known, one would be almost tempted to suppose that Thalamoplana was derived 

 from an ancestral form possessed of" a large number of penes, which are here to be 

 seen in process of reduction. 



The three genera Semonia* Discocelis,^ and Thalamoplarta may conveniently be 

 grouped in a distinct sub-family of the Leptoplanidee, characterized by the possession 

 of marginal eye-spots, of a large blunt penis, and by the absence of an internal prostate 

 gland. Semonia, like Discocelis, has a common genital atrium, whilst Thalamoplana 

 resembles Discocelis in possessing external prostatic glands and a bilaterally 

 symmetrical accessory vesicle connected with the vagina. 



Leptoplana gardineri, n. sp. 



One specimen, collected by Mr. Gardiner in 1899. 

 Colour in spirit specimen uniform yellowish-white. 



Measurement. Length, about 16 millions. ; breadth, 7 "5 millims. Buccal opening 

 sub-central. Female aperture about 6 millims. from the hinder end of the body. 



* Von Plehn, 'Jen. Zeitsehr. f. Naturwissenschaft,' Band XXX., pp. 157-159, Taf. XL, figs. 5, 12; 

 Taf. XIII., fig. 3. See also Laidlaw, ' Proc. Zool. Soc.,' 1903, p. 309. 

 t See especially Lang, he. cit., Taf. XIII., tig. 1. 



