﻿DUMORTIERIA. 233 



descendants must have continued to accentuate this feature until, on their outer 

 whorls, something resembling the distant ribbing of the inner whorls of Dum. 

 prisca was produced. Secondly, either simultaneously or subsequently, a quicker 

 mode of coiling must have been adopted ; while thirdly, either simultaneously or 

 subsequently — it is impossible to say which without examining the inner whorls of 

 Dum. prisca, a small carina must have been produced, while the ribs must have 

 become obsolete on the ventral area. 



Such are the changes necessary to produce Dum. prisca from Oatull. Vernosse ; 

 and therefore Dum. prisca is a more developed form than Oatull. Dumortieri ; but 

 the evolution of either may be said to be analogous to the production of Caloceras 

 from forms possessing the Planicostan abdomen. 1 



Dum. prisca, Dum. costula, Dum. sparsicosta, and Dum. Levesquei may all be 

 said to be analogous to Caloceras ; and the inner whorls of the other species of 

 Dumortieria show their connection with these forms. These other species, 

 however, assume, when adult, a totally different appearance — the effect, solely, 

 of continued lateral compression of the whorl ; but this causes a convergence 

 between Grammoceras and Dumortieria. Dum. striatulo-costata, radians, Moorei, 

 &c, all show this Grammoceratan-like-stage, and only differ from one another in 

 that the more they are developed the earlier they assume this stage. Unlike Gram- 

 moceras, these species of Dumortieria have apparently taken on this stage without 

 passing through what may be called the Arietan stage, 2 that is, a period of sulcate 

 and carinate ventral area. The inner whorls of Dum. striatulo-costata (PI. XL) 

 seem to indicate this fact clearly. 



It is very curious that, while the smooth ancestral form splits up into two 

 branches, one of which at the commencement of the Liassic period developed into 

 Caloceras {Ophioceras), the other should have waited until the commencement of 

 the Middle Lias, and should then have started on a career of similar changes. 

 But the resemblance is even more peculiar than this. While Caloceras evolved 

 the Arietidse., first by producing a carina on its ventral area, and then by gradually 

 broadening its ventral area — adding a furrow on each side of the carina, — the 

 same thing should happen in certain species of the later Catulloceras- and Dumor- 

 fo'erm-branches. But so it is. Catulloceras Vernosse has an uncarinate ventral 

 area, Dum. prisca and Dum. Levesquei add a small carina, while Dum. arata and 

 Catulloceras Dumortieri have developed small furrows each side of the carina 

 in the same manner as obtains in the Arietidse. Still more curious than this, how- 



1 " The crossing of the abdomen by the pihe . . . is common in the young of Caloeerau forms " 

 (Hyatt, " Genesis of the Arietidse," ' Smithsonian Contributions to Knowledge,' vol. xxvi, p. 142). 

 It may therefore be concluded that the abdomen crossed by pila?, which Hyatt has previously tersely 

 called " the Planicostan abdomen," was a feature of the adult ancestors of Caloceras. 



2 In Grammoceras the Arietan-stage may be said to be obsolescent; in Dum. arata and Catull. 

 Dumortieri are signs of nascent Arietau-like characters. 



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