244 Lieut. -Col. H. H. Godwin-Austen on new 



structures, such as muscles, elteoblast, and probably also the 

 genital organs. The distal portion of the left coelomic sac, 

 which preserves the form of a tube, becomes the pericardium. 

 This grows forwards, soon reaches the anterior portion of the 

 germinal disk, and, becoming expanded like a club at its 

 anterior end, assumes the shape which is already sufficiently 

 well known from Kowalewsky's description. The lower wall 

 of the expansion of the pericardial sac, which adjoins the 

 endoderm, is differentiated tolerably early as a thickened 

 plate, which represents the rudiment of the heart. The 

 heart itself, which is formed by the invagination of this 

 plate, is not completely developed until the period of the 

 formation of the cyathozooid. 



With this I conclude these brief notes on the earliest stages 

 in the development of Pyrosoma, and may summarize the 

 chief results of my investigations as follows : — 



(i.) The embryo of P?/rosowa is formed from both fertilized 

 and unfertilized elements, since not only the blastomeres, but 

 also the kalymocytes, take part in the formation of the 

 cyathozooid. 



(ii.) In the differentiation of the germinal layers the nuclear 

 mass first divides into two portions — an ectoderm and a meso- 

 endoderm ; of these the latter further differentiates into a 

 multilaminar mesoderm and a unilaminar endoderm. 



(iii.) The mesoderm first appears in the form of two typical 

 ccelomic sacs. 



(iv.) Of the two coelomic sacs the left alone undergoes 

 further development, and is subsequently differentiated into 

 an axial mesoderm and a pericardial tube ; whereas the right 

 sac breaks up into separate cells, which are afterwards dis- 

 persed through the body of the cyathozooid. 



Odessa, March 1890. 



XXIX. — On supposed new Species of Land-Mollusca from 

 Borneo helonging to the Genera Opisthostoma and Diplom- 

 matina. Bj Li'eut.-Col. H. H. Godwin-Austen, F.R.S., 

 F.Z.S., &c. 



[Plate VII.] 



In the paper on Bornean Cyclostomaceee published in the 

 * Proceedings of the Zoological Society,' 1889, p. 332, 

 I recorded and described all the species that were then 

 known to me. Since that time I have received another small 



