DE. T. DAVIDSON OX EF.CEXT BRACHIOPODA. 2:5 



lie minutely describes and illustrates the arrangement of the spicula in Terebratullna caput- 

 serpentis as well as in other genera in which they occur (sec Pi. IV. figs. 10, 11). lie 

 tells us that each of the lobes of the mantle in this species has four vascular sinuses, of 

 which the lateral ones are more developed than the median two, which last contain 

 in their interior the organs of reproduction. The spicula cover these sinuses, and 

 afterwards line the walls of the visceral cavity, producing two large convex surfaces, 

 which become united and thus completely close this cavity. In the dorsal valve the 

 mantle reflects itself on the brachial apparatus, formed here of calcareous laminae bent 

 in the shape of a ring, and spread themselves on the labial appendages, of which they 

 line the interior of the great canal, as well as the cirri ; the cirri, he adds, which I 

 have also carefully examined in many specimens, are stated by Hancock to be large, 

 much-branched, colovu-less, glass-like, and pellucid, somewhat like the antlers of a deer, 

 only the branches are all on the same plane and are flattened and depressed a little ; 

 that they are, however, frequently much complicated, forming a central network with 

 irregular radiating branches ; in others, again, the branches pass from a single network 

 centre. The spicula lie in the outer layer of the inner lamina, and are crowded to such 

 a degree that the edges of the branches are almost in contact, thus forming an extensive 

 though incomplete network of calcareous matter over the trunks of the great jiallial 

 sinuses ; elsewhere they are sparingly distributed towards the margin of the lobes, where 

 they are rather numerous. The brachial disk is trilobed, the central lobe elongated and 

 spirally convoluted. 



In 1771, in the ' Fundamenta Testaceologiae ' it is represented as the animal of Terehru- 

 tulina caput-serpentis without its shell. In 1773, Pennant describes the same species, and 

 gives a figure of its labial appendages. Grundler, again, in 1774, devotes a whole 

 plate to enlarged figures of the same appendages, and in two of them points out the 

 position of the loop. Since that period they have also been described and illustrated 

 by several malacologists, and more particularly by Albany Hancock, in his classical 

 memoir ' On the Organization of the Brachiopoda' (1858). Mr. Hancock informs us, 

 at p. 793 of his work, that, " To facilitate the examination of the soft parts, it is neces- 

 sary to reduce the valves by dissolving them in dilute acid. When the calcareous matter 

 has entirely disappeared, and the calcified shell-membrane has been removed, the trans- 

 parent tegumentary envelope of the animal is exposed to view ; and this, when placed 



in water, assumes the form of the shell Towards the posterior or umbonal 



region the enlarged extremities of the shell-muscles are always apparent, clustered 

 together on each side of the antero-posterior line, forming reddish-coloured patches." 



The muscles naturally divide themselves into two groups, the valvular and those for ad- 

 justing the shell to the peduncle. As the arrangement of the muscles is stated by Hancock 

 to be essentially the same in all the articulated Brachiopoda, and they have been described 

 further on in JFakUieimkt flavescens, it will not be necessary here to refer further to those 

 details, although it may be added that Hancock mentions an important modification as 

 occurring in T. caput-serpentis, which consists in the large and powerful dorsal adjustor 

 muscles not being attached to the hinge-plate, as in W. Jlavescens, but having their 

 insertions in the valve itself. The posterior extremities of these muscles are seen on the 



