DR. T. DAVIDSON ON EECENT BRACHIOPODA. 167 



their origin, where they arc sustained by tlie two lunye-processes, or oral lamina.', the 

 points of which reach as far forward as their external margins. They fill up the greater 

 portion of the pallial chamber ; and in their arrangement accurately resemble the cal- 

 careous spirals of Atrijpa reticularis, a Silurian fossil, only their approximate sides are not 

 flattened. The arm throughout is composed of a slightly depressed tube or canal, carrying 

 along its outer margin the semi-cartilaginous grooved edge, bearing the fringe of cirri as 

 in W. auslrails. The brachial fold in front of the groove is largely developed, and com- 

 pletely overlaps it. The tube or the great brachial canal terminates at the side of the 

 oesophagus in a delicate membranous sac of no great extent, which projects into the 

 perivisceral chamber, as first noticed by Prof. Huxley . . . 



" The parietes of the great canal are somewhat stouter than in W. australis ; the mus- 

 cular fibres, however, are arranged in the same manner as in it, but are more numerous, 

 particularly the longitudinal ones, which form a well-defined band towards the proximal 

 extremity of the arm .... A large development of the brachial apparatus seems 

 necessary in the economy of the animal, and the various ways in which it is folded up 

 and disposed within the pallial chamber are only so many methods of arranging within a 

 limited space the requisite extent of organ." 



Mr. Hancock seems to doubt the unrolling and projection of the arms in Hhynchonella 

 or that the arms arc in any way instrumental in opening the valves. 



" In H. psiltacea [p. 814] the disposition of the alimentary canal is the same as iu the 

 Terebratulida}. The oesophagus is, however, considerably longer than in Waldheimia, the 

 mouth approaching much nearer to the ventral valve. The liver is larger than usual, 

 and the hiliary secretion is conveyed through the lateral walls of the cardiac extremity 

 of the stomach by four short ducts, two at each side, one being placed a little in advance 

 of the other. 



"The intestine is rallier long and gradually tapers downwards. On reaching the 

 ventral valve, directly behind the extremities of the occlusor muscles, it turns backw^ards 

 and upwards, and, detaching itself from the mesentery, advances a little, and ter- 

 minates in a much enlarged, rounded extremity, which inclines to the right or 

 left, varying in this respect in different individuals. The termination projects freely 

 into the centre of the perivisceral chamber, and here, as in the TerehralHlldoi, there 

 is no anus ; the bulbous enlargement is entire, exhibiting no opening whatever. . . . 

 In BhynchoHella the gastro-parietal and ilio-parietal hands, jiarticularly the latter, are 

 longer than in Wcddheimia .... The reproductive organs [p. 818] have much the same 

 disposition iu Rhi/nchonella psittacea [as in the other Terebratnlidce], but the dorsal 

 genital sinuses arc not connected with the so-called vascular trunks, while the ventral 

 are. The genital band, too, which is yellow, is much more closely convoluted, having 

 sometimes almost the appearance of being fused into one mass, the interstices being just 

 sutficient to permit the passage of the muscular ties, which are stout, and are arranged in 

 imperfect longitudinal and diagonal lines. These ties give a granulated or pitted 

 appearance to the ovarian impression in the shell, and are worthy of the attention of 

 the palaeontologist. . . . 



" There are two of these oviducts In all the Brachiopods that have come under my 



