part 4] EVOLUTION OF THE LlPAROCEKATID.i;. 26-> 



here, since its evolution is different from that of the series described 

 above. Phricodoceras includes the taylori group of ammonites : 

 these forms rapidly acquire bituberculation or even trituberculation, 

 without any marked expansion of the whorl such as is characteristic 

 of the genera hitherto dealt with. 



Mr. Buckman has called my attention to a specimen of Phrico- 

 doceras lamellosum (A. d'Orbigny) from the armatus zone of 

 liadstock, in his collection (Xo. 2917), in which the strongly tuber- 

 culate stage is followed by a stage with feeble ornament and a great 

 increase in the size of the whorl, the outer whorls thus bearing 

 some resemblances to the involute forms of the preceding genera. 

 It is distinguished by 



(1) The greater elevation of the outer whorl, 



(2) Enlargement of the whorl subsequent to the acme of ornamentation, 



(3) The close approximation of the outer tubercles to the ventral margin. 



Ammonites striatus bicornis Quenstedt (4, pi. xxviii, fig. 24) 

 and A. taylori Quenstedt (4, pi. xxvii, fig. 19) show a similarly 

 expanded outer whorl with cata genetic ornament. 



VI. Study of Species and Developmental Details. 

 A. Lipaeoceeas. 



LlPAEOCEEAS SPAESICOSTA, Sp. UOV. (PL XXI, figs. 2 rt-2 b & 



3 a-S e ; text-figs. 2 a-lg, p. 266.) 

 Dimensions of holotype : — 



Diameter. Whorl height. Whorl thickness. Umbilicus. 



35 mm. 32 per cent. 35 per cent. 44 per cent. 



A small Capricorn ammonite, with depressed whorl ; strong but 

 widely-spaced ribs, feeble on the venter. Until the diameter of 

 6 mm. the whorls are smooth, stout ribs separated by wide concave 

 spaces appearing at that diameter. Thirteen ribs on the outer 

 whorl, of which the last nine have paired tubercles; periphery broad, 

 with the low arch characteristic of the genus. 



At 22 mm. diameter the suture is moderately simple, but shows 

 the feature of the «*enus, namely : the first lateral lobe is wider 

 than the external saddle, and about equal in depth to the external 

 lobe, thus part of the first lateral lobe is ventral to the outer line 

 of tubercles (text-fig. 2y, p. 266). 



The type-specimen was collected by Air. L. Richardson, F.G.S., 

 from a well at Queen's Wood Cottages, Prestbury, near Chelten- 

 ham. Specimens may also be obtained at a similar horizon at 

 NaptoD (Warwickshire), and at Dumbleton (Gloucestershire). 



Development. (PI. XXI, figs. 3«-3<°.) — In studying the 

 development several small specimens collected at Xapton were 

 used. In this locality fine specimens of the capricorns and of the 

 li&terocjeiics stage of Liparoceras may be found, fortunately with 

 the inner whorls well preserved in calcite. 



y2 



