1927 SYSTEMATIC 



might be either D. strumatum or D. variocostatum one would suppose ; 

 but it might be another species : the finds were only fragmentary. 

 The evidence, however, seems to point to a stout Dichotomoceras some 

 15 to 20 feet below D. dichotomum. For the present, then, it may be 

 sufficient to date all the species of Dichotomoceras as hemera dichotomum, 

 keeping in mind the possibility that the stout forms may mark an 

 earlier date than the thin forms. 



Some further notes on specimens of the genus may be given. In 

 the holotype of D. variocostatum, which is in the Buckland Collection 

 in the University Museum, Oxford, the earliest whorls are well exhibited : 

 they are polygyral and smooth, that is, ribless, but carry rather marked 

 nodes at intervals : such nodes may be relics of ancestral parabolae. 



The primary ribs of this specimen regularly bifurcate — Miss Healey 

 noticed this character in her description ; but in D. strumatum the ribs 

 quite frequently trifurcate. In D. strumatum there are fewer ribs to 

 whorls than there are in D. variocostatum — due, in the former species, 

 to preparation for, and elaboration of, bigger ribs. This is shown below. 



The ultimate whorl in D. dichotomum (1) is the true last whorl 

 of the specimen, body-chamber being practically complete. The ultimate 

 whorl in the other examples is the last whorl which they happen now 

 to possess, more or less of body-chamber having been lost. 



The thicknesses of D. dichotomum (2) should be less than those for 

 No. 1 ; but also they are without allowance for the attrition (suffered 

 in the Boulder Clay ?). Narrow, upstanding, laminar ribs are a special 

 feature of the genus in the pre-swo lien-ribbed stage. As such ribs 

 stand up some z\ to 4 mm. a possible difference of some 5-8 mm. in 

 measurements would considerably affect proportions. 



Harpoceratoides, S. Buckman, 1909, T.A. i, p. ii\ "Type, Am, 

 alternatus, Simpson, No. 9 " (PI. IX). The figure given in PL IX, H. alter- 

 natus, and what is figured as H. strangwaysi in PL DCCXXXIX, show 

 comparative graphs of some divergence, so far as height and umbilical 

 proportions are concerned, while the thickness shows the former 

 (H. alternatus) to be more than 7 per cent, stouter than the latter. 

 Both species have the undulations of the inner area, a sort of bunched 

 or false primary ribbing breaking up into many secondaries — a character 

 possibly peculiar to the genus. The same character is seen in Harpoceras 

 kisslingi, Hug, (Abh. Schweiz. Pal. Ges., xxv, 1898, p. 14 ; iv, 2), which 

 was claimed as Harpoceratoides (T.A. i, 1909, p. ft) : in ornament it 

 would seem to differ by having broader and fewer secondary ribs. No 

 information is given by its author as to its thickness. However, there 

 appears to be a simple method of distinguishing the three named forms 

 by graphs. In each species the height of whorl per cent, is, at first, 

 greater than the umbilication ; later, the two are equal; later, umbilication 

 is greater than height. The critical point, therefore, or the cross-over 

 point where both are equal, is to be noted. Graphs yield the following 

 results : in H. alternatus equality would be at a diameter of 172 mm. ; 

 in H. strangwaysi (PL DCCXXXIX) it is at 125 mm. ; in H. kisslingi 



