292 Royal Society. 



The arrangement of the genitalia in Rhynchonella is very remark- 

 able. The sinuses have the same arrangement in each lobe of the 

 mantle. The single trunk formed by the union of the principal 

 branches in each lobe opens into the inner and anterior angle of a 

 large semilunar sinus which surrounds the bases of the adductors, 

 and opens into the visceral cavity. The floor of this great sinus is 

 marked out into meshes by the reticulated genital band, and from 

 the centre of each mesh a flat partition passes, uniting the two walls 

 of the sinus, and breaking it up into irregular partial channels. 



There are the same anastomosing bands uniting the gastro-pa- 

 rietal and ilio-parietal bands on the stomach in Rhynchonella as in 

 Waldheimia, and a pyriform vesicle of the same nature, but I did not 

 observe in Rhynchonella those accessory vesicles upon the origins of 

 genital bands, which I observed once or twice in Waldheimia. 



I could find no trace of arteries terminating the elongated, ovoid 

 and nearly straight ' ventricles' of Rhynchonella ; their ends appeared 

 truncated, and as I have already said, repeatedly presented a distinct 

 external aperture. 



Such appear to me to be the facts respecting the structure of the 

 so-called hearts in the Terebratulidce ; what I believe to be an import- 

 ant part of their peripheral circulatory system, has not hitherto, so 

 far as I am aware, received any notice. 



In Waldheimia the membranous walls of the body, the parieto-in- 

 testinal bands and the mantle, present a very peculiar structure ; 

 they consist of an outer and an inner epithelial layer, of two corre- 

 sponding fibrous layers, and between them of a reticulated tissue, 

 which makes up the principal thickness of the layer, and in which 

 the nerves and great sinuses are imbedded. 



The trabecular of this reticulated tissue contain granules and cell- 

 like bodies, and I imagined them at first to represent a fibro-cellular 

 network, the interspaces of which I conceived were very probably 

 sinuses. Sheaths of this tissue were particularly conspicuous along 

 the nerves. On examining the arms, however, I found that the oblique 

 markings, which have given rise to the supposition that they are 

 surrounded by muscular bands, proceeded from trabecular of a simi- 

 lar structure, which took a curved course from a canal which lies at 

 the base of the cirri (not the great canal of the arms, of course) 

 round the outer convexity of the arm, and terminated by breaking 

 up into a network. These trabecular, however, were not solid, but 

 hollow, and the interspaces between them were solid. The network 

 into which they broke up was formed by distinct canals, and then, 

 after uniting with two or three straight narrow canals which ran 

 along the outer convexity of the arm close to its junction with the 

 interbrachial fold, appeared to become connected with a similar 

 system of reticulated canals which occupied the thickness of that 

 fold. 



It was the examination of the interbrachial fold, in fact, which 

 first convinced me that these reticulated trabecular were canals ; for 

 it is perfectly clear that vessels or channels of some kind must sup- 

 ply the proportionally enormous mass of the united arms with their 



