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the dorsal side, the centre of the dorsum again begins to round 

 out, but traces of the primitive dorsal furrow remain in the de- 

 pressions on either side of the central, gibbous dorsal face formed 

 by this outgrowth, as in Fig. 24. 



This gibbous face is immediately suppressed when the whorls 

 come into contact, and its transient appearance can only be 

 accounted for as due to the genetic tendency of the paranepionic 

 whorl to resume the gibbous metanepionic form of dorsum as soon 

 as the pressure resulting from the abrupt curve is slightly relieved.* 



Trocholites ammonius, Hall. 



This species, of which the collection of the Museum of Compara- 

 tive Zoology possesses a very large number, collected by Mr. C. D. 

 Walcott, has a very peculiar, rough, fretted surface, and only very few 

 specimens show longitudinal lines such as are described and figured 

 in T. planorbiformis by Conrad. This surface is due to the minute 

 crenulations or waves in the outlines of the projecting edges of the 

 laminae of growth. When these are wide enough apart one can 

 distinguish crenulated transverse lines ; when too close they inter- 

 fere and the regularity and continuity of the lines are broken into 

 a multitude of more or less discontinuous, short lines. Sometimes 

 a network of lines is formed by the regularity of the intersection 

 of the crests of the crenulations in successive laminae. This cuticu- 

 lar ornamentation is so easily destroyed that it is often present only 

 on parts of the same specimen. 



Longitudinal lines may be seen through it, but, as stated by Hall 

 and observed by the writer, these are rarely present in the New 

 York specimens. They do, however, sometimes exist all over the 

 abdomen and sides, and are well defined in specimens in which the 

 cuticular corrugations are absent. 



The lines of growth are extremely crowded, and what are called 

 the costae occur at wider intervals and more irregularly. They are 

 probably the traces of former apertures. These are more promi- 

 nent in some specimens than in others, but never seem to have the 

 aspect of true fold-like costations. 



The lines of growth form deep, broad sinuses on the venter; 

 rise into lateral crests on the sides, sinking towards the lines of 

 involution, and forming a sinus in the contact furrow. These are 



*This opinion would be more convincing, if it were not for the fact, that in Oranoceras 

 similar transformations occur in an adult cynocerau form of the Devonian. 



