Natural Science 



A Monthly Review of Scientific Progress 



April 1899 



NOTES AND COMMENTS. 



An Ancient Mollusc. 



The genus Pleurotomaria is the oldest among Gastropod Molluscs, and 

 begins its career in the oldest fossiliferous strata, the Lower Cambrian, 

 with the earliest known Trilobites and Brachiopods. 



In geological times its race was abundant, more than 1150 species 

 having been described, and it attained its maximum, about 370 species, 

 in Jurassic days, since which period it has steadily declined in numbers. 



The genus was long held to be extinct, but in 1855 the first recent 

 example was obtained from a great depth between the islands of Marie 

 Galante and Dominica in the West Indies by Commandant Beau. 

 Unfortunately this shell appears to have been tenanted by a hermit- 

 crab, so that the animal still remained to be discovered, and it was not 

 till 1871 that the living occupant was obtained by A. Agassiz when 

 on the " Hassler " expedition. Since then other examples of the genus 

 have been obtained from the West Indian region and off Japan, that, 

 with odd specimens of unknown locality, bring the total number of 

 known specimens up to twenty-one belonging to four species. In only 

 six instances, however, has the animal been preserved, and five of these 

 have been procured on American dredging expeditions. 



Dr. Dall has published a few details concerning the animals of 

 two, P. quoyana and P. adansoniana {Bull. Mils. Comp. Zool. Harvard, 

 vol. xviii. 1889), and another specimen of P. quoyana has lately 

 been entrusted by Prof. A. Agassiz to Messrs. Bouvier and H. Fischer 

 to make as thorough examination as its preservation permitted of the 

 soft parts. The result of their investigations now lies before us 

 (Arch. Zool. expe'r., Ser. iii. tome vi. pp. 115-180, pis. x.-xiii.). The 

 animal in question had been greatly damaged in extraction from the 

 shell ; the whole of the spiral visceral-hump, with the gills and a great 

 part of the mantle, were wanting. Under these circumstances it is 

 hardly remarkable that the authors' observations are mainly confined 

 to two points — the radula and the nervous system. 



The radula is remarkable for the number and variety of the teeth 

 in each transverse row, and the curvature of these rows. On either 

 side of the median tooth in each row 117 teeth, and the rudiments of 



18 NAT. SC. VOL. XIV. NO. 86. 257 



