1G2 DE. T. DAVIDSOA^ ON EECENT BEACHIOPODA. 



the glass slides are shaken between which the embryos are placed they become ruptured, 

 and the cells and granulations that they contain escape. 



In the next stage which M. Lacaze-Duthiers found in the same group of young, the 

 embryo no longer presented a uniform mass, but was divided into two halves, one of 

 which, more voluminous than the other, Avas in every case found attached to the tipper 

 filament. 



After many details which I have not space to reproduce, M. Lacaze-Duthiers con- 

 cludes by saying (p. 324) : — On the lower face, an embryo in the more advanced state 

 shoAvs four eye-shaped spots, visible in the thickness of its anterior intermediary lobe. 

 A distribution of yellowish matter recalls tolerably well the origin of the liver in the 

 Acephala or the Gasteropoda ; it is probably in the middle of these small agglome- 

 rations of yellowish matter that the stomach is excavated, and one may already recognize 

 in these series of little bundles the lobules and secretory glands of the liver of the adult 

 TheckUum. We have reproduced on Plate XXIII. a few of M. Lacaze-Duthiers's admir- 

 able illustrations of the embryo with two and four lobes. 



92. Thecidium Baeeetti, Woodward, MS. (Plate XXIII. figs. 9-11.) 



Thecidium Barretti, Davidson, Geol. Mag. vol. i. p. 17, pi. ii. figs. 1, 2, 3, 1864 ; Crosse, Journ. cle 

 Concli, 3" ser. t. vi. p. 272, 1866; Dall, Amer. Jouru. of Couch, vol. vi. p. 151, 1870, Bull. Mus. Comp. 

 Zool., Harvard, vol. iii. p. 42, 1871. 



Shell small, somewhat pyriformly ovate, attached to marine ol)jects by a portion of the 

 back of its beak. Dorsal valve semicircular, flattened, and slightly convex at the umbo. 

 Ventral valve somewhat pyriform, very convex, deep, and thickened ; beak mode- 

 rately produced ; area triangular, but more or less irregular on account of the position 

 and extent of the attached surface. The interior of the dorsal valve is slightly concave' 

 with an oblong, square-shaped, prominent, cardinal process between the dental sockets. 

 A broad, thickened, raised, granulated margin encircles the valve, and forms a bridge 

 over the small, deep, visceral cavity, close to the basis of the cardinal process ; the gra- 

 nulations are larger and more prominent on the inner margin. On reaching the front, 

 near the middle, the margin suddenly cm-ves upwards on each side, and unites so as to 

 form a central A-shaped ascending process {[/), the attenuated extremity of which is direc- 

 ted towards the middle of the bridge- shaped process {h) already described in TheckUum 

 medUcrraneum (see PL XXIII. fig. IG). The descending apparatus (/, k) is united close 

 to the extremity of the central angular ascending process, and follows at a little distance 

 the curves of the inner margin of the same ascending process. Length and breadth 

 about 2 lines. Colour white. 



Hah. This small and well-characterized species was obtained by Lucas Barrett at 60 

 fathoms off the north-east coast of Jamaica, and was found fossil by him in the newest 

 Pliocene beds of that island. 



Ohs. In external shape Thecidium Barretti cannot be distinguished from the Mediter- 

 ranean species ; but its interior is very different, and resembles, in its simple arrange- 

 ment, that of several Jurassic forms, such as Thecidium Moorii, T. triaiKjulure, &c. 



