part 4] evolution of the lipakocekati]>j:. 273 



Differs from other species of Liparoceras in having stout whorls 

 and involute form accompanied by unpaired fine tubercles. 



Occurrence. — Upper part of the iheos zone, Dorset. 



L. pseudostriatum is similar in appearance to Androgynoceras 

 divaricosta, but has a stouter whorl, smaller umbilicus, unpaired 

 tubercles, and more complicated suture of a different pattern. 



The holotype is in the collection at University College, Cardiff. 



B. ^Egoceeas. 



^Egoceilis aff. latcecosta (Sowerbv). (PL XXIII, rigs. Sa & 3 6; 

 text-fig. 5, p. 274.) 



Compare Ammonites latzecosta Sowerby. 1 



Dimensions : — 



Diameter. Whorl height. Whorl thickness. Umbilicus. 



72 mm. 32 per cent. 33 per cent. 47 per cent. 



50 30 30 46 



Slender whorls little embracing, coiling regular ; on the outer 

 whorl are about thirty slightly-curved ribs, sharp on the sides and 

 less 'prominent on the venter. On the last half -whorl, striae are 

 present between the ribs, especially on the venter. Feeble bituber- 

 culation commencing. Suture fairly complicated, external saddle 

 very wide, first lateral lobe narrower and trifid, symmetrical. 

 Collected by Dr. W. D. Lang (Coll. No. 1566) in the Lower Lime- 

 stone (Jatceeosta sub-zone) of Dorset. 



Development of JEgoceras Jatceeosta. (PI. XXIII, 

 figs. 4r/-4(/.) — The development was studied in a small specimen 

 (W. D. Lang Coll. No. 534) from the latcecosta sub-zone (?), 

 Stonebarrow. The young specimen, at a diameter of 14 mm., 

 shows a body-chamber equal to half a whorl. 



The protoconch is of normal form, preserved in calcite : an 

 examination by transmitted light shows the so-called ' prosiphon ' 

 of Munier-Chalmas (PI. XXIII, fig. 4d). This structure is not 

 always to be seen in ammonite protoconchs, owing to differences in 

 preservation, and several investigators have failed to find any trace 

 of it. Recently, however, Grandjean has shown it to be a solid 

 structure extending from the end of the siphon to the inner wall 

 of the protoconch. 2 The ' prosiphon ' of JSgoceras Jatceeosta 

 resembles the second type described by Grandjean, consisting < f 

 one long ' bandelette,' thus differing from JE. planicosta, in which 

 it is relatively short. 



Shell-development proceeds normally, and, until &. Jatceeosta 

 attains a diameter of over a millimetre, it is scarcely to be distin- 

 guished from other voung capricorns. At a diameter of 4 mm. 

 (PI. XXIII, fig. 4a) the whorl height of M. Jatceeosta is less than 



1 ' Mineral Conchology ' vol. vi (1829) pi. dlvi, fig. 1. 



2 F. Grandjean. ' Le Siphon des Ammonites, &c.' Bull. Soc. Gc'ol. France, 

 ser. 4, vol. x (1910) p. 49G. 



