984 



PALEONTOLOGY OF NEW- YORK. 



AtHTEIS SPIRJFEKOTDES. 



Central portion of the spires.* 



The origin of the crura, or point of attachment to the hinge-plate, is 

 indicated in the figure at a, from Avhich the two lamella3 proceed for a 

 short distance in a nearly direct line forward, but are soon bent upwards 

 and recurved upon themselves as shown in the figure at b, whence they 

 are again bent downwards into the cavity of the dorsal valve. From this 

 point the lamellae follow very nearly a direction parallel to the external 

 contour of the shell, being the exterior bands indicated by the dotted 

 lines to c. Farther on, these become expanded and send off from each one 

 a projecting process at d, and thence are united in a solid plate at e, 

 forming the loop which connects the two parts of the spiral arms. It will 

 also be observed that the lamellae are twisted, the exterior part at d 

 becoming the interior at the point of junction of the two parts. This 

 plate, formed by the junction of the lamellae, is sharply bent backwards 

 almost in the plane of the longitudinal axis of the shell ; and thence ris- 

 ing nearly at right angles, becomes bifurcated at/, giving origin to the 

 accessary lamellae g, which are recurved in a plane essentially parallel 

 to the first volution of the spire, and coalesce with it at the points indi- 

 cated by the dotted lines h, thus acting as an additional support to the 



*I am indebted to the rare mechanical skill and artistic manipulation of Mr. R. P. Wuitfield for 

 the preparation of specimens illustrative of this and another species of Atiitris, as well as of tho 

 spires of Mebistella, ctc> 



