162 INFERIOR OOLITE AMMONITES. 



Now, Dumortieria Levesquei is in general shape and appearance not unlike 

 Grammoeeras striatulum, — an extraordinary fact when we remember how dissimilar 

 are the representatives of these two in the Jamesoni-zone. The gradual passage 

 from Dumortieria Levesquei into Dum. Moorei (Lycett), which may be expressed by 

 a compression of the sides, and a decrease in the coarseness of the ribs, is 

 parallelled exactly by the gradual mutation from Gramm. striatulum to Gramm. 

 mactra — a mutation which is accompanied by a similar decrease in the coarseness 

 of the ribbing, and also by a decrease in the size of the carina. Thus the conver- 

 gence between Grammoeeras and Dumortieria, which is close in the Jurense -zone, 

 becomes in the Opalinum-zone — between Grammoeeras mactra (Dumortier) and 

 Dumortieria Moorei (Lycett) — a truly extraordinary instance of the ultimate 

 resemblance of two species sprung from different ancestors. 



To distinguish Grammoeeras from Dumortieria is never an easy matter; while 

 to distinguish Grammoeeras mactra from Dumortieria Moorei is a task requiring 

 the very greatest care, so extreme is the convergence. However, the following 

 general rules will be found to hold good in the separation of the two genera : 



Grammoeeras. Dumortieria. 



Eibs with a slight bend on the lateral area, Eiba with a general forward inclination, with- 



and with a very pronounced forward sweep on out any bend on the lateral area, and with only 



the ventral area. a short forward projection on the ventral area. 



Suture-line with short lobes and shallow Suture-line with a fairly deep siphonal saddle, 



saddles, with a well-marked inferior lateral lobe, with a superior lateral lobe, generally longer than 



and with the inner portion of the suture-line level the siphonal, with practically no inferior lateral 



with the rest. lobe, but with the inner portion of the suture-line 



brought down very obliquely. 



The above rules will serve our purpose for the present, but they will receive 

 elaboration when I treat of Dumortieria. The reader must remember that only 

 by constant practice will he become expert enough to separate the two genera at 

 a glance. 



The genus Grammoeeras is interesting for its geological range, during 

 which its progression in the matter of development is very slight. The 

 earliest species known at present is Grammoeeras antiquum (Wright) in the 

 Jamesoni-zone ; but as this shows in its flattened sides and absence of ribbing 

 more development than many of the later forms, we can easily come to the 



matter, and, as Dr. Haug has done much work with these forms, my remark must be received with 

 caution. The suture-line of Bum. Levesquei agrees fairly with the sutures of young Dum. Jamesoni, 

 as shown by Haug, and points to their being descended from a common stem, but I prefer to imagine, 

 unless the evidence to the contrary is very conclusive, a gradual increase in the suture-line from 

 Polymorphites to Dumortieria Levesquei, and not a quick increase to Jamesoni and a subsequent 

 reduction to Levesquei unaccompanied by whorl-changes sufficient to account for it. 



