240 SYNOPTIC COLLECTION. 



Pelseneer, who has made a special study of the anat- 

 omy of these forms, admits that " the division established 

 on the very empirical character of the presence or the 

 absence of a shell, is quite justified by the anatomical 

 differences." 1 



Thecosomata. The Thecosomata are among the more 

 primitive forms. They possess a shell, and the animal 

 adds new layers to it as it grows. The head is indistinct 

 and the wing like organs extend from it on either side 

 and are joined at the anterior edge above the mouth. 

 The anus is on the left side. 



It may be that there are primitive forms among the 

 straight, cone-shaped Pteropods living to-day, but judging 

 from our present knowledge of the anatomy of these 

 straight forms, one must consider them as secondary and 

 specialized members of the Pteropod group, and the spiral 

 forms living to-day as the more generalized. 



The spiral condition is represented by Spirialis aus- 

 tralis Soul. (No. 551; PI. 552). Its tiny spiral shell 

 protects a spirally twisted body and the aperture is 

 closed by an operculum. 



Spirialis rostralis Soul. (No. 553 ; PL 554), has a nauti- 

 loid shell with an umbilicus. The figures exhibit the 

 forward part of the body with its appendages, the ex- 

 panded wings, and the viscera enclosed by the mantle. 



Creseis acicula Rang (No. 555, alcoholic specimen; 

 Nos. 556, 557, shells; PL 558, figure of animal in shell), 

 is provided when young like all Pteropods with a shell 

 formed by the everted shell gland. In most members of 

 the group this shell disappears and a secondary shell is 

 formed. In Creseis, however, the primitive and second- 

 ary shells are both retained throughout life. The adult 

 shell is smooth, cone-shaped, and bilaterally symmetrical, 

 with a simple aperture. 



It would seem that this bilateral symmetry was evi- 



1 Chall. Rep., Zool., XIX, part I, 1887, p. i. 



