585 
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 Cyv¢. Murchisonia 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, 2. ferversum, 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 Ammonoidea., 
PROC. AMER. PHILOS. SOC. xxxII. 148. 3V. PRINTED JULY 18, 1894. 
