1926 SYSTEMATIC 15 



The species figured by Dr. Neaverson as Paravirgatites kimmeridgensis 

 (Amm. Kimm. 1925) is unlikely to be a Paravirgatites : the ribs are 

 much too straight and regular, as may be seen by comparison with 

 P. paravirgatus, T.A. CCCVIIIb ; but as Dr. Neaverson has given no 

 suture-line, the right genus cannot yet be determined. 



Shotoverites, S. Buckman, 1925, T.A. DLXII. Like Wheatleyiies, 

 but with decidedly shorter lobes and with more distinct tricostulation 

 in the magnicostate stage. Shotoverites rednctus should replace Wheatley- 

 ites rednctus (CCCLXXXIV), whose date would probably be later than 

 W heatleyites , namely pringlei, on the evidence of S. pringlei and its 

 locality. 



Kerberites. T.A. V, 1924, DXX, Genotype Kerberites kerberus. 

 Heavy-ribbed, with, frequently, triplicate secondaries, whence the name, 

 from the three-headed dog Kerberus. The whorl-section is obovate, 

 that is, somewhat convergent. Specimens have been figured from 

 [Chicksgrove, Tisbury], and from the Cockly Bed of Swindon, Wiltshire. 

 Poor examples are known from Long Crendon (North-West pit), Bucks, 

 which, by their matrix and the extent of quarry opened, come from 

 about the Rubbly Limestone Bed, Behemothan 7, (T.A. IV, 1922, 26). 

 The matri.x shows that they cannot be lower, and the extent of the quarry 

 that they might only be a little higher. 



The noticeable point about the example now figured, PI. DXXa, b, 

 (T.A. VI, 1926), is the preservation, in the inner whorls, up to a 

 diameter of about 35 mm., of the small and close-set costate or virgatite 

 stage. Yet there seems to be no other difference, neither in suture-line 

 nor in plotted proportions, from the holotype (T.A. V, 1924, DXX), 

 which has well-costate inner whorls. Either the holotype has accelerated 

 the oncoming of magnicostation — precedentive palingenesis, or the 

 plesiotype has retarded it unduly — cunctative palingenesis. 



The phenomenon of the plesiotype's inner whorls may justify two 

 statements. First, that the magnicostate Gigantids are the descendants 

 of multi-parvicostates — either of virgatites, in which secondary ribs 

 are bundled into a primary, or of pectinates, in which the ribs are like 

 those of a fine tooth-comb, only not always single, sometimes bifurcate. 

 Second, that it is dangerous to make a distinct species, let alone a different 

 genus, on account only of dissimilarity of the early stages, for such 

 dissimilarity may be only the result of differential acceleration or 

 retardation along a given line of development. 



The phenomenon of the inner whorls of the holotype, as well as of 

 the inner whorls of other Gigantids, teaches that the change over from 

 parvicostation to magnicostation in such forms could be so rapid that 

 the parvicostate stage was practically omitted from the recapitulation : 

 as soon as ornament began to be formed, magnicostation started. 



Lydistratites shows magnicostation arrived at after a virgatite stage ; 

 Wheatleyiies exhibits magnicostation developed after a pectinate stage ; 

 in these two cases the difference of inner whorls would justify differentia- 

 tion into genera. Because the two forms of inner whorls are not sequent 

 to one another ; they are parallel developments even when not strictly 

 synchronous. Rapid acceleration of the magnicostate stage in species 

 of either of these two genera would yield inner whorls looking very 



