348 



brancbialis tenuissima, reflexa, incumbens. Diatn. |; alt. ^polL 

 Hah. Puget Sound, Oregon. 



It has the same form and color and surface as T. vitrea, but 

 is more rounded in front, the incurved areas of the beaks much 

 broader and flatter, the ligament aperture larger, &c. Like 

 other species of the genus, it varies much in form, some being 

 quite globose, while others are compressed. 



Mactra debilis. T. parva, alba, tenuis, transversa, elongato- 

 ovata, antice ovata, postice acuta, valde hians ; valvis tumidis, 

 postice compressis ; umbonibus eminentibus acutis, antemedia- 

 nis : cardo debilis ; dente V-formi parvo ; dentibus lateralibus 

 Curtis, acutis. Long. If ; alt. 1 ; lat. ^ poll. Hah. Singapore. 



A very delicate species, resembling in all its outlines small 

 specimens of Mya arenaria. 



Mr. Girard read a paper on the development of Plano- 

 cera elliptical in which he traced the egg through a series 

 of successive periods. 



1st. The period of evolution — from the origin of the egg to 

 the moment when it is laid ; it originates like organic cells, 

 generating its mass around a new centre of attraction till it is 

 capable of leading an independent life ; during this time the egg 

 depends upon the mother. 2nd. Period of preparation — from 

 the laying of the egg till the manifestation of the embryo ; the 

 vitelline mass divides, and prepares its substance for the future 

 being ; during this time the egg is immovable, but is connected 

 with the parent. 3d. Period of the embryo — the vitelline 

 sphere moves within the egg-membrane, assumes various forms, 

 and then escapes. 4th. Period of the larva — the young animal 

 moves freely about, assuming different shapes, till it is trans- 

 formed into a mummy-like body. 5th. Period of the chrysalis — 

 differing as widely from the larva as in the butterfly ; the analogy 

 is complete in its external features. 



He places PlanaricE rather among Mollusks than among 

 Articulata. 



Prof. Agassiz had some time since observed that the 



