154 LIVING FOSSILS. 



the Mollusca, among Ti'liicli the gastropods, described under 

 the name of pleurotomaria, afford the most striking- 

 instance. The shells of this genus, which are frequently 

 very elaborately sculptured, have a general external resem- 

 blance to a Ti(rbo or a Trochvs. but are readily distinguished 

 by a deep horizontal slit in the middle 

 of the outer Tvall of the mouth, from 

 which the genus takes its name. The 

 genus Pleurotomaria was originally 

 established in the year 1826 on the evi- 

 dence of a sjsecies from the English 

 lias ; and a host of other extinct kinds 



■p gg g]^j,]j p£ were subsequently described, ranging 



Plenroiomaria. from the Silurian to the chalk. Till 

 1855, no one, however, had the least idea 

 that the genus was still existing, but in that year a living 

 specimen was oljtained off Mariegalante, wliicli was subse- 

 quently sold in London in 1875 for £26. The species to 

 which this first recent sp>ecimen belonged was named 

 P. qunyana ; and three examples of the same form have 

 subsequently been obtained. The next discovery of a 

 Pleurotomaria occurred in 1861, when an imperfect speci- 

 men of another si)ecies (P. adansonia) was obtained. A 

 second example of this same species was taken in 1882 

 near Guadaloupe, and three others are in existence, while 

 a seventh was purchased in 1890 at Tobago. The latter 

 example was a very large shell, measuring just under six 

 inches in height ; and it was also distinguished by its 

 striking coloration, being marked by a number of oblique 

 splashes of reddish-orange on a pale flesh-coloured 

 ground. A third species of the genus (P. heyrichi) was 

 obtained from Japanese waters in 1877, and three 

 examples have been subsequently secured. Pinallv, the 

 fourth and largest living species is only known by a 

 single examjjle, which was recognized in 1879 among a 

 collection of shells at Rotterdam, and is believed to have 

 come from the Moluccas. The height of this fine shell is 

 six and three- Cjuarter inches. 



It will thus be apparent that only fourteen specimens of 

 living Pleurotomaria;, referable tfi four distinct species. 



