BRAcnioroDA. 499 



lining membrane inwards. The generative organs, in the male, as 

 in the female, developed in the ventral lobe of the mantle, commence 

 — if we may term the part next the hinge of the shell their beginning 

 — by a loop on each side of that lobe, situated at the point of bi- 

 furcation of the two sinuses ; these loops are shown in CCCIII.,^^. 1, 

 pi. 3, the inner layer of the mantle, with the adherent tunic of the 

 sinus, being reflected to shoAv the looped portions of the ovaria (12, 12). 

 In the dorsal lobe the generative organ commences at about the same 

 distance from the hinge, by a simple obtuse extremity, and follows, 

 as it advances, the ramifications of the sinus in which it is lodged. 

 There are thus four distinct ovaria or testes, two in each mantle lobe ; 

 those of the ventral lobe being doubled, or bent upon themselves, 

 near the cardinal-attached border of the lobe. It may be presumed 

 that the embryos developed under the influence of their suspension in 

 the aerated and nutrient fluids of the pallial sinuses, escape, at a certain 

 stage of that development, by dehiscence, at or near the free ciliated 

 border of the pallial lobes. The course and phenomena of this de- 

 velopment would form a most interesting and acceptable subject to 

 a competent microscopical observer favourably situated for pursuing 

 it throughout the breeding season of the Terehratulce. I have ob- 

 served the following stages in ova taken from the ramified pallial 

 ovarium of a Lingula, preserved in spirits. CCCIII , pi. I, Jig 7, a, 

 shows an impregnated ovum, in which the germinal vesicle or vesicles 

 have disappeared, and the germ-mass has been formed, occupying the 

 entire ovum, which has assumed an oblong form : a peripheral stratum 

 of the derivative germ-cells was more compact and of a somewhat 

 lighter colour than the central mass. Ih. b, shows the formation of 

 a smooth membrane, probably covered by ciliated epithelium, around 

 the germ-mass : lb. c is a transverse section of such an ovum, show- 

 ing its triedral figure : lb. d is an ovum further advanced, with the 

 rudiment of a peduncle : lb. e, an embryo with the peduncle more 

 produced. I could distinguish no organs in these embryos of the 

 Lingula : there was no trace of shell. Thus the Lingulce are pro- 

 vided with a peduncle before they quit the parent. 



On comparing together the existing genera of Brachiopods we 

 may perceive that the modifications which are traceable in their 

 respective organisations bear relation to the different situations 

 which they occupy in the sea. 



The Lingula, living usually near the surface, and sometimes where 

 it would be left exposed by the retreating tide, if it were not buried 

 in the sand of the shore, must meet with a greater variety and 

 abundance of animal nutriment than can be found in the deeper 

 waters, where the Terebratula usually resides. Hence its powers of 



K K 2 



