DAVIDSON — ON SOME FOSSIL BRACHIOPODA. 459 



the bottom of the shell, divided by a small longitudinal ridge, may be 

 clearly seen the impressions left by the adductor longus anticus of Owen 

 (anterior occlusor of Hancock) ; but those of the posterior occlusor could 

 not be distinctly seen on any of the valves that have come under my 

 examination. It is probable that the occlusor muscle had four attach- 

 ments in this valve, as in Terebratula and other genera. On each side 

 of the hinge-plato above described, and of its aperture, exists a pro- 

 longed testaceous plate, situated at a higher level, and forming the 

 ledge or rim of the inner socket-walls. These two prolongations 



become lengthened, and give 

 origin (while serving as points 

 of attachment) to the two 

 lamellae («), which, by their 

 convolutions on either side, 

 constitute the spires, as well 

 as the intermediate process by 

 which they are united. The spi- 

 ral lamellae, disposed vertically 

 to the plane of the valve, con- 

 verge at first, then bend sud- 

 denly upon themselves, at a 

 short distance from their 

 origin, outwards and back- 

 wards {h) in the shape of a 

 half - circle, which, passing 

 first close to the sockets (5), then follow in the direction of the bottom 

 of the valve {o, c) to become recurved again, and thus producing the 

 first of the seven or eight convolutions of which each cone is composed. 

 From the first two spiral lamellae spring up perpendicularly other two 

 secondary ones (o, d), which, by being bent, become united towards the 

 middle of the shell {d) between the two spiral cones, and afterwards 

 form but a single branch (from dioc). This last, after having attained 

 the upper level of the spire (at e) becomes again bifurcated, and forms 

 other two lamellae (/) recurved in half-circle, and terminating back- 

 wards by a free extremity {g) between the first and second coil of each 

 spiral cone. 



Such, at least, are the dispositions of the spiral and other parts 

 in AtJiyris pectinifera, and are probably the general dispositions of 



2 L 2 



