BRACH10PODA. 695 



great length. The mouth occupies the same position relatively as in 

 the two former types, and is lodged in a brachial groove bordered by 

 a lip. Lip and groove extend to the tips of the arms, together with a 

 row of cirri, viz. that which corresponds to the convex side of the horse- 

 shoe of Argiope. The cirri are, however, numerous, and are arranged 

 along the whole or part of the length of each arm in double file 1 . The 

 apex of the spiral is turned towards the aperture of the shell in Lingula ; 

 towards the dorsal valve in Rhynchonclla among living Testicardines, 

 and in a few extinct, e.g. Atrypa. The spiral skeleton supporting the 

 arms shows that in the extinct Spirijeridae the apex of each spiral was 

 turned outwards laterally, in Coelospira (Atrypidae] inwards, towards its 

 fellow, and in the K oninckinidae towards the ventral valve. The 

 lophophore can be protruded in Rhynchonella, and to a slight extent in 

 Lingula (Morse), but apparently not in other Brachiopoda. It is often 

 supported by a calcareous skeleton, which takes the form of two ridges on 

 the inner aspect of the dorsal valve in extinct Productidae (Neumayr) ; 

 of two simple processes projecting freely, one from each side of the dorsal 

 hinge plate, e. g. Argiope ; of two rods or crura similarly placed, e. g. 

 in Rhynchonella ; of two crura prolonged towards the margin of the valve 

 where they recurve upon themselves towards the ventral aspect, and are 

 prolonged back again towards the base of the shell, where they are usually 

 united by a transverse bar, e. g. Waidheimia, some Terebratulae ; or finally, 

 of a calcareous spiral attached to each of the crura as in the extinct 

 Spirijeridae, &c. A cross bar usually unites the bases of the two spirals. 

 There is much variety in the skeleton supporting the lophophore of 

 Terebra tulidae. 



The free surfaces of the mantle-folds, body, lophophore and peduncle, 

 are covered by a unilaminar ectoderm, the cells of which vary in different 

 regions. They secrete an extremely thick laminated cuticle on the 

 peduncle. The surface of the mantle-folds applied to the valves of the 

 shell is also covered by a layer of ectoderm cells in Lingula and Crania, 

 but in the Testicardines the place of the epithelium is taken by a thin 

 membrane (?). The ectoderm of the inner and lateral aspects of the cirri, 

 of the brachial groove, and the aspect of the lip turned towards the groove, 

 and in Crania of the mantle, is ciliated. The edges of the mantle-folds 

 are in most instances only free for a short distance, so that the valves of 

 the shell can be opened but to a short extent. The free edges are pro- 

 duced into a fold so as to form a circumpallial groove at the bottom of 



1 The preceding description of the lophophore in the text is based on a comparison of recent 

 memoirs and such specimens as were accessible with Hancock's figures and descriptions. Compare 

 the following figures in his Memoir, Ph. Tr. 148, 1858; PI. LV, fig. i, Waldheimia australis; fig. 

 2, Terebratula caput serpentis ; fig. 3, Rhynchoiiella ; PI. LVI, figs. 2 and 3, Waldheimia australis ; 

 PI. LXV, figs. 6, 7, 8, Lingttla. 



