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Trocholites,* but is usually less and occurs later and more slowly 

 and it is not an absolute decrease. That is to say the outer whorl 

 never falls off so "much in the ratio growth as to become actually 

 smaller than the inner volution in any of its diameters until the 

 gerontic stage. In this stage the falling off in the rate of increase 

 by growth may and sometimes does accomplish this result on the 

 last part of the outer whorl. 



A description of the parallelism of different genetic series and 

 the constant and often repeated tendency that these exhibit to 

 evolve a series of similar forms has been given in the Introduction. 

 This tendency produces straight, arcuate, loosely coiled and close 

 coiled, and finally involute shells in each group, however distinct 

 they may be in structure. 



The tendency to bend towards the side opposite the hyponome is 

 almost universal in all shell-covered Cephalopods. There are a 

 few arcuate species that bend towards the hyponome like Barrande's 

 Cyrtoceras nitidum, but many even of his group of the so-called 

 " endogastrica " have, like his Cyrt. Mu r x hi 'som 'a and Cyrt. neutrum, 

 the hyponome and therefore the true venter on the outer or convex 

 side. There is only one genetic series or genus, as a whole, 

 that appears to contradict this statement. All of the species of the 

 true Phragmoceras except one, P. perversum, Barrande, bend 

 towards the ventral side and about all have the siphuncle and also, of 

 course, the azygos sinus of the hyponome in the aperture and the 

 corresponding sinuses in lines of growth on the same side. The 

 shells of this genus are much compressed and the apertures are very 

 much elongated and present a unique aspect. They are contracted 

 along the central parts and the hyponome or motor organ is 

 removed as far as possible from that part of the aperture which 

 must have given opportunity for the external extension of the arms. 

 This fact, however, is counterbalanced by the aperture of P. perver- 

 sum, this being an extreme case of differentiation and removal as 

 widely. as possible of the hyponomic and brachial sinuses of the 

 apertures and yet the shell is bent towards the dorsum and the 

 siphuncle is ventral. Many species of Gomphoceras (Acleistoceras) 

 are bent ventrally, whereas others with similar apertures and charac- 

 teristics are bent dorsally. So far, therefore, as the characters of 

 the apertures go, it is not possible to state that the bending is inva- 



*This peculiarity has led some authors to suppose that Trocholites had a protoconch 

 like that of the Amruoiioidea. 



