62 THE NAUTILUS. 



cheon angular instead of keeled, and which was named Lutraria 

 ventricosa by Gould in 1851. The radial ridges on the escut- 

 cheon are individually variable and the worn valve for which 

 Morch (Nal. Blatt. 1861) proposed the name of subalata was a 

 specimen of alata either derived from ballast or otherwise trans- 

 ported from the Atlantic coast of Costa Rica. 



A new species from the west coast of Mexico and belonging to 

 the group of M. alata is described below. 



The Mactrella (Harvella) elegans Sowb,. is one the most beau- 

 tiful and fragile bivalves of the family. Confounded with Rceta 

 canaliculate, by early writers, it was referred to the Florida fauna 

 in the Conchologia Iconica. This error, and a certain varia- 

 bility of outline common to most Mactridre, led Conrad in 1867 

 to propose the name of pacifica for the supposed Pacific coast 

 form. But there is at present no evidence that such a shell 

 occurs in Atlantic waters. 



The known distribution of the American species of the group 

 is as follows : 



ATLANTIC WATERS. 



MACTRELLA ALATA Spengler. 



Porto Rico, Jamaica, Nicaragua, Costa Rica, the north coast 

 of South America and south to Ceara, Brazil. 



MACTRELLA IHERINGI Dall. 

 Santos and Sao Paulo, Brazil. 



PACIFIC WATERS. 



MACTRELLA EXOLETA Gray. 



Mazatlan, West Mexico, to Panama and Guayaquil. 



MACTRELLA CLISIA n. sp. 



Manzanillo, West Mexico, to Santa Elena, Ecuador. 



MACTRELLA (HARVELLA) ELEGANS Sowerby. 

 Panama. 



MACTRELLA CLISIA n. sp. 



Shell white, extremely arcuate, with the silky periostracum 



