686 Modification of the Ai^ex in Gastropod Mollusks. 



to be of specific value (Oliva, Yoluta) while in others almost no 

 variation exists (Natica, Litornia, Neritina). The study of the 

 apex shows us to what great extent variation takes place after 

 the birth of the mollusk, and how its environment may affect the 

 young mollusk to a specific extent. I venture to ask, therefore, 

 if we cannot trace many of the slight specific differences found in 

 shells, which have caused naturalists to disagree regarding their 

 stability, to this fact of post-embryonic development affected by 

 changes of environment ? May we not, in fact, ascribe great 

 specific variation to this cause, and may it not also be true that a 

 species born off the Florida coast and carried by currents to the 

 Massachusetts coast might become in the latter place a different 

 species or variety from what the same individual might have been, 

 had it lived in the former locality ? These are questions which 

 have presented themselves to the writer in the course of his 

 studies of this subject, and they seem to him to be worthy of in- 

 vestigation. 



In the present paper the apices of 132 species are described, 

 embracing many of the families of the marine gastropods. 



Family OLIVID^. 

 Genus OLIVA Bruguiere. 

 OliTa literata Lamarck. (Fig. 1.)* 



Apex smooth, shining, consisting of three and a-half whorls, 

 which rapidly increase in size ; the third whorl is twice as high as 

 the second and the fourth whorl is of the same height, but wider; 

 a carina begins at the second whorl and encircles all the volutions 

 a little above the suture of the whorl below; it is faint upon the 

 second whorl, but becomes very strong on the third whorl. On 

 the older whorl a deep channel is formed between the carina and 

 the whorl below. The color of the apex is very like spermaceti. 

 Four specimens examined, which showed no variation. 



Oliva reticularis Lamarck. (Fig. 2.) 



Apex smooth, shining, consisting of two whorls, of which the 

 first is very large, exceeding the second by two diameters ; the first 

 whorl is rounded, knob-shaped and without a carina, the latter be- 

 ginning on the second whorl, near the suture, and gradually grow- 

 ing stronger as the shell increases with age. The color and texture 

 is like spermaceti. Compared with literata this apex is much 



* The accompanying plates are numbered XVIII-XX. [Ed.] 



