D. M. S. Watson — A New Genus of Brachiopods. 215 



of the right adductor is often in close contact with the dental area in 

 the region where the implacement should be. 



The visceral surface of the shell is usually smooth, but in certain 

 individuals has a pustulose surface exactly resembling that which is 

 usually found on the visceral surface of the shells oi; Strophalosia. 

 The shell shows no signs of being punctate, and apparently consists 

 of a single layer. Apart from the ridges it is extremely thin, perhaps 

 seldom exceeding - 1 mm. 



The preceding description contains a purely objective statement of 

 the facts shown in my specimens of this Brachiopod. From them it 

 is possible to draw certain conclusions as to the structure of the 

 dorsal valve and the animal's structure and habits. 



It seems certain from the whole structure that the dorsal valve 

 covered only the space included within the raised flange and that, 

 when closed, its edge rested in the rabbet on the inner face of that 

 rim ; the margins of the shell would thus be uncovered all round, as 

 they must certainly have been in the region posterior to the hinge- 

 line. This disposition of the dorsal valve shows that the flange is 

 really the edge of the ventral shell, the areas lying outside it being 

 merely of the nature of callosities developed for more perfect fixation. 



The fact that only a single asymmetrical abductor muscle can be 

 recognized suggests that the two valves could not be opened widely, 

 if at all, by a strict hinge action at the hinge-line, and the equally 

 marked disparity between the adductors can only be explained by 

 supposing that the action of the two kinds of muscles together was 

 to slew the dorsal valve round on the ventral so as to bring the slits 

 in the dorsal valve, which correspond to the loops of the flange of the 

 ventral valve, over the areas between the loops, thus allowing a free 

 circulation of water. 



The differences between the very small and the large specimens 

 show that the loops originally develop as notches on the edge of the 

 dorsal valve by a failure to grow at certain spots on the edge of the 

 mantle. The whole structure is, in fact, developed by an emargination 

 of the edges of the shell similar to that which divides the valves of 

 Bilobites or Pygope into such distinct halves. 



From the very small unbroken area which remains inside the shell 

 it seems certain that the lophophore could not have been spirally 

 coiled as it is in most Brachiopods, but must have been persistently 

 larval as it is in the Thecideidse and Argiope, passing in front of the 

 month and forming a decollated band following the edge of the 

 dorsal valve. 



For this Brachiopod I propose the name Poikilosakos petaloides, 

 gen. et sp. nov. The original of PI. XIV, Fig. 1, is the holotype. 



Poikilosakos at once recalls Keyserlingina from the Uralian beds of 

 Russia. From Tschernyschev's description of casts of the visceral 

 surface of the ventral valve, which alone are known, it is obvious 

 that the two forms resemble one another in having a large ventral 

 valve, and a much smaller dorsal one whose edge is dissected and 

 fits into a raised ridge on the ventral valve, which is thrown into 

 quite similar loops in the two genera. 



The text-figure given by Tschernyschev of the real appearance of 



