50 FAUNA OF THE CORNBRASH. 



Type. — This species affords a striking example of the evil of not selecting a 

 definite specimen as type, but uniting under one name several specimens which 

 are believed to be related as young and adult. In this case the specimens were 

 first referred to two species of Sowerby, to neither of which they belong. Later, 

 D'Orbigny discovered his mistake, and renamed one of the species as above, but 

 gave no separate description of the new name, consequently it has to be gathered 

 from the figure. At a later date Oppel renamed the specimens which had been 

 referred to the other Sowerbyan species as funatus. We are thus thrown back 

 for the type on D'Orbigny 's figure and Quenstedt's description. D'Orbigny says 

 the ribs become very regular and die away on the outer third and are replaced by 

 four to six riblets on the back, becoming less marked and even effaced along the 

 median line. All but the larger lateral ribs are lost at a diameter of 74 mm. ; there 

 are two or three transverse furrows per whorl. Aperture, when complete, shows a 

 large ear-shaped process, narrow at the base and much enlarged and oblique at its 

 extremity. Quenstedt only says that the whorls " have their greatest breadth, not 

 in the middle of the side, as in the ornati, but close to the umbilical edge " — a 

 character which makes the least fragment recognisable. D'Orbigny's type is from 

 Niort, but it is not certain whether his specimen is referred to the Great Oolite or 

 to the Lower Oxfordian — and probably, therefore, to the Cornbrash. 



Description. — The principal specimen on which the occurrence of this species in 

 the Cornbrash is founded, has compressed whorls only slightly overlapping and 

 leaving exposed in the umbilicus seven or eight of the earlier whorls. The sides 

 are somewhat flattened in the centre. They commence with an elevated, quickly- 

 turned umbilical border and gradually decrease in thickness to the rounded 

 periphery. The earliest whorls are much more rounded, and gradually become 

 flatter. In the first whorl the surface is nearly smooth ; in the second begin 

 irregular ribs, certainly resembling in this respect the Oxfordian P. bakerise ; 

 in the third there are 35 uniform rounded ribs, in the fourth 41, in the fifth 

 45, and in the sixth 40, the outer portion commencing to show the divided 

 peripheral ribs. Here the body chamber is reached, and on it the ribs become 

 gradually more separated. The outer ribs are uncovered towards the end of the 

 sixth whorl, but they do not obviously spring out of the inner ones, but leave a 

 ribless space between them, having a slightly forward slope. In the remaining 

 quarter of the body whorl there is a lesser rate of increase, and the ribs are more 

 spaced and the riblets disappear. The sides are contracted and produced into 

 auricles. Here and there a rib and its corresponding posterior concavity are more 

 pronounced, producing the characteristic constrictions of the genus. The shell is 

 fairly thick, but when it is thinner there is a distinct intermission along the 

 median line, where is a feeble raised band bounded by two parallel depressions. 

 The septal part of the shell is so completely crystalline that no sutures can be 

 made out. 



