Spiral 



Sat lira I 



angle. 



(tii(jle. 



20" 



101'^ 



part 4] THE EV0LUT10]V OF LIASSLC UASTllOPODS. 'U7 



near Cheltenbaiu (Mr. L. Uicluirdsoii's collection, L.Gr. 45). 

 Similar specimens were found at the same horizon in a railway- 

 cutting at Toddington, near Wincheombe, Gloucester (L. Kicliard- 

 son coll. L.Gr. 42 a & L.G. 39). These latter forms differ from 

 Procerifhiuiu cf. numismale in having shallower sutui'es and a 

 single spiral thread within the sutural depression. Tlie three rows 

 of nodules are ap])roximately equal in size, the anterior row, however, 

 may be stronger than the posterior set, the reverse of that which 

 occurs in P. cf. numlamalc. Tate's specimen of P. 7iu)nis)nalG 

 was found in i\\Q Jameson i zone of Cheltenham, and appears (from 

 his description and the rather poor figure) to be almost identical 

 with Mr. Richardson's specimens. 



PiiocERiTiiiUM (Exelihsa) cf. 1 NEiiALiASicuM (Cossmann). 



VL Kxelissa oifraliasica CossiiKUiii (3, \>\. v, fig. 1, and j)!. vii, lig. 2U). 



Dimensions of our specimen. 



Ijencjth. lireacUh. Lcncjth of spire. 



4'4 nun. 10 per cent. 73 per cent. 



A small pupoid form having slightly convex whoi'ls ornamented 

 with strong axials, which are faintly curved, crossing live un- 

 equally spaced, spiral threads. Oral margin simple. 



I)e velopmen t. -'^I'lie ,young of this form has three rows of 

 nodules by the llfth whorl, increasing to four on the seventh 

 whorl ; all later whorls haye five rows. 



It differs from l)i'. ( .ossmann's species in that it never develo])S 

 seven spirals, and the whorls are not quite so convex. It may be 

 considered a parallel development to P. numismale, but on a 

 somewhat different line of descent, probably from a more advanced 

 form resembling P. sla//ari. 



Locality and horizon. — Our specimen is from the Lower 

 Lias (jamrsoni zone) of Toddington. near Wincheombe, Gloucester 

 (L. liichardson coll. L.G. 427;).' 



Notes on the Generic Name Exelissa. 



The name E.velissa was proi)osed by E. Piette to cover some- 

 what pupjcform Cerithia in which the aperture is entire, the last 

 whorl cylindrical and slightly contracted at the base. Numerous 

 species have been referred to this genus by l^iette and subsecjuent 

 writers, an examination of ligures of which shows that they include 

 forms with many different variations in the character of the 

 ornamentation ; and it is here suggested that puj)oid forms of 

 ProcerithidiC hitherto referred to JbJ.velissa are mei'ely catagenetic 

 species of .Proccnlhmm, which have been developed inde})endently 

 at numerous horizons. ' This conclusion is deduced from the fol- 

 lowing facts : — 



(1) There is great diversity in the ornamentation of species referred to 

 Exelissa. On the whole, the pupoid species in the Lower Lias have 



Q. J. G. S. No. 308. 2 A 



