Zoological Society. 303 



the mantle lobes take their origin : they may be regarded as the 

 branchial veins conveying the aerated blood to the two hearts, 

 which ace situated exterior to the liver, and just within the origin 

 of the internal calcareous loop : they are accompanied in their course 

 by much smaller vessels, probably the branchial arteries. Such is 

 apparently the system of respiration in Terebratula. 



The viscera occupy a very small space near the hinge. The ali- 

 mentary canal commences by a small puckered mouth, situated im- 

 mediately behind the folded extremities of the arms. It passes back- 

 wards, and expands into a membranous stomach, surrounded by the 

 liver, a bulky gland of a green colour and minute follicular tex- 

 ture, which communicates with it by many orifices. The intestine 

 passes down to the hinge, and then turns to the right side and ter- 

 minates between the two mantle-lobes. No trace of a salivary gland 

 was found. 



The generation of Terebratula is that of the ordinary Bivalves. 

 In two of the larger specimens the ova had insinuated themselves 

 between the layers of the mantle, and partly surrounded the 

 branchial vessels. When so far advanced they obscure the organi- 

 zation of the mantle which adapts it for respiration : this organi- 

 zation is consequently most satisfactorily observed in very young 

 individuals. 



Mr. Owen describes in detail the muscles, the arms, and the pe- 

 culiar internal testaceous apparatus or loop connected with the 

 hinge and supporting the arms. In the species which he examined, 

 with the exception of Ter. psittacea, he finds that the loop possesses 

 some elasticity, and when acted on by the muscles becomes in its 

 reflected part sufficiently convex to press upon the perforated 

 valve and separate it slightly from the opposite one ; thus compen- 

 sating for the absence of the thick arms of Lingula, which in their 

 protrusion push open the valves, and also for that of the elastic 

 fibres constituting the ligament of ordinary Bivalves. 



The Orbiculce examined by Mr. Owen consist of specimens of Orb. 

 lamellosa, Brod. 



Along the whole circumference of the valves shining cilia are 

 seen projecting for an extent varying from 2 to 4 lines : they are 

 consequently much longer than in Terebratula and in Lingula anatina, 

 and are rather longer than in Ling. Audebardii, Brod. On exami- 

 nation under a high power they are observed to be beset with 

 smaller setce, which probably gives them greater power in determi- 

 ning the respiratory currents. The mantle is similarly vascular to that 

 of Terebratula, there being, in the upper lobe, four principal trunks 

 (comparatively, however, much shorter than in that genus); and 

 two in the loner. These trunks terminate in sinuses, situated close 

 to two strong tendinous membranes, which circumscribe the visceral 

 mass, and to which the mantle-lobes firmly adhere. Here the veins 

 of both mantle-lobes join, and the common trunk or sinus passes ob- 

 liquely through the membrane, and may be plainly seen distributing 

 ramuli. over the liver and ovary. 



The muscles and viscera form a rounded mass, situated in the pos- 



