74 THE CONSTITUTION OF THE GASTROPOD PROTOCONCH, 



discoid ancestors, whilst A. tentoriforme should come from 

 trochiform ancestors." 



I have now no doubt that such a conclusion is at fault. 

 I would say rather that the former species was truly inter- 

 mediate between the trochiform and the open-coiled species of 

 the ^4. iniqjeriale type, and that, in its early Jeanic stage, it 

 carries traces of an ancestry common to both, but whose track 

 is already eliminated from the ontogeny of A, tentoriforme. 



Another instance of an interesting neanic shell, which, in 

 this case, owes its form to its ancestry will be found below, in 

 the account of the probable course of the evolution of Cyma= 

 tiurn jpar kmsomanu DL Perry, from the Tertiary species, C tor- 

 tirostris Tate. 



Boutan has discussed the phylogenetic significance of the 

 stages in the ontogeny of FissureUa, and his figures have been 

 reproduced in several places. It is interesting to note that 

 the embryonic stages which he figures have been fixed, as it 

 were, in other genera. 



On Plate i., a series of retouched photographs of (Jymatium 

 'parkinsonianum Perry, and C . tortirostris Tate, are repro- 

 duced; similar photos, of C. ahhoti Tate, are also reproduced 

 for comparison with these. The interest attached to these 

 shells is that Cymatiufii j^f^''^^^^ 'Ionian urn is apparently the 

 recent form of C . tortirostris. The typical form of the latter 

 (fig. 2) retains its nodulose ribs and well-developed revolving 

 lirse throughout. In the variety (fig. 1), which has the same 

 outline as the above, the revolving lirse are obsolescent on the 

 last half of the body-whorl, and the last few ribs are less 

 nodulose. In Cymatium parkin soniaiiu in (figs. 5-8), the early 

 (neanic) whorls are heavily lirate and nodulosely ribbed (fig. 

 8), exactly resembling the early whorls of the typical (■yw<(- 

 tium tortirostris, but in the last three whorls the lirae are 

 reduced in size and the ribs not nodulose. The contour is 

 that of C . tortirostris. The main dift'erence between the two 

 (for the species are alike), is that, in the recent form, the 

 reduced sculpture appears earlier in the shell. The recent 



