Vol. 54.] MISS J. DONALD ON THE GENTJS ACLISINA. 49* 



round it on nearly the same plane. The protoconch does not always- 

 adhere to the highest whorl of the conch, but sometimes stands nearly 

 erect (as in PI. Ill, figs. 3a, 36) ; at other times it is but slightly 

 (PL ly, figs. 13a, b, c) or not at all raised ' (PI. IV, figs. 14a, h) from 

 the spire, which it occasionally overhangs (PI. lY, figs. 12 «, h). The 

 protoconchs of all the specimens that I have examined are invariably- 

 smooth, but some are too much worn to show any signs of orna- 

 mentation, even if it had originally existed. Where the apex is fairly 

 preserved, the junction of the protoconch with the conch is marked 

 by a faint rib or varix ; and the ornamenting threads begin almost 

 immediately, and are similar in character to those on the adult, 

 though less numerous. Whether this posterior part of the conch, 

 or whether the varix at the junction, represents the brephic stage, 

 it is diflficult to ascertain ; at any rate, it is interesting to note the 

 early development of the characteristic ornamentation. Eoth 

 Dr. Jackson ^ and Mr. G. F. Harris ^ observe that characters similar 

 to those of the adult frequently appear in the brephic stage. 



Resemblances. — The genus which this most resembles is Streptacis^ 

 Meek ^ (non Stre-pta.vis, Gray), of which Str. Whitfielcli, Meek, is the 

 type. They are similar in general form, in having sigmoidal lines 

 of growth, and in possessing an irregular protoconch. They difi'er in. 

 Streptacis having perfectly smooth whorls instead of being orna- 

 mented by spiral lines and grooves, and also it is not stated whether 

 the inner lip is reflected on the body-whorl as in Aclisina. These 

 two genera are, however, evidently closely allied. 



Aclisina is very like Aclisoides in general appearance, and where 

 aperture, lines of growth, and protoconch are not preserved, it 

 is often difficult to distinguish between the two genera ; the outline 

 of the outer lip and the protoconch are seen to be quite distinct in 

 well-preserved specimens. In the character of the outer lip Aclisina 

 resembles Loxonema, but it differs from that genus in being orna- 

 mented by spiral threads, in having more convex whorls, and also 

 in the form of the protoconch. 



From the typical Turritellidse the genus under consideration may 

 be distinguished by having more convex whorls, and by having the 

 aperture of a different shape. Prof. E. Koken,^ however, believes it 

 to be the direct forerunner of certain Turritellce. It certainly bears 



^ It is a question whether these specimens with the protoconch entirely 

 attached, and coiled on the same plane as the rest of the spire, should be grouped 

 with the typical Aclisina. Though they agree with the genus in general 

 appearance, the lines of growth are not preserved on any of them, and it is 

 possible that they may be more closely allied with the subgenus Bhahdospira, 

 which has straighter lines of growth. We are, however, ignorant at present of 

 the form of the protoconch and aperture of Rhahdospira. 



^ ' PJiylogeny of the Pelecypoda,' Mem. Boston Soc. Nat. Hist. vol. iv, no. 8, 

 1890, p. 290. 



3 ■ Oatal. Tert. MoUusca Brit. Mus. (Nat. Hist.),' pt. i. Australasia, 1897, 

 p. xiii. 



^ ' Descr. of New Sp. of Foss. from Ohio & other Western States & Territories,*^ 

 Proc. Acad. Nat. Sci. Phil. 1871, p. 173. 



^ ' Ueber die Entwickelung der Grastropoden vom Cambrium bis zur Trias,'" 

 Neues Jahrb. Beilage-Band vi (1889) p. 458. 



Q. J. G. S. No. 213. E 



