OBSERVATIONS ON THE INTERNAL APPENDAGES OF THE 



GENUS ATRYPA; 



Witli a Notice of the Discovery of a Loop, connecting tlie Spiral Cones. 

 By R. p. IVIIITFIELD. 



The internal appendages of the genus Atrypa [Balman), have long 

 l)een known to consist of a pair of sjiiral cones, placed side by side, with 

 their ajjices directed towards the cavity of the dorsal valve. The lamellaj 

 forming these cones have been described and figured as having their origin 

 on the socket-walls of the dorsal valve, then suddenly deflected oui,wards, 

 running parallel with the inner margin of the valve. At the point where 

 they are abruptly deflected, is figured a pair of short, pointed, crural pro- 

 cesses, directed toward each other, but not conn(?cted. 



This appears to be all that has hitherto been known regarding these 

 appendages; although, judging from analogy, we might reasonably sup- 

 pose that, as most genera of Brachiopods having spiral cones have been 

 foujid to possess some sort of loop or crural process connecting them, tliat 

 Atrypa would not be destitute of some similar arrangement. 



By carefully cutting and preparing favoraMe specimens, I have found 

 that in place of the short crural processes so often figured, there is an entire 

 and continuous loop connecting the spiral cones, in a very similar manner 

 to that shown by Prof. Hall to exist in his genus Zygospira, but having 

 its connection with the spiral ribbons, at a point relatively much nearer to 

 their origin on the hinge-plate; still UKjre distant, however, than the points 

 figured by Mr. Davidson and others. This loop, so far as yet observed, 

 is confined to the rostral or posterior part of the shell, and never passes 

 over or in front of the spires as in Prof. Hall's genus. From its origin 

 on the posterior portion of the first volutions of the spires, the loop curves 

 gently forward and upward; the central or elevated portion lying between 

 and behind the cones, and forming a more or less abrupt curve, or pro- 

 hmged into a point directed towards the dorsal valve. In some specimens 

 where the origin of the loop is on the outer portion of the first volution of 

 the spire, the ribbon of the loop, and that of the spire, are parallel for a 

 short distance, that of the spire lying uppermost. Where the origin is 

 near the attachment to the hinge-plate, the loop has the appearance of 

 being a continuation of the principal band; while the junction of the crura 

 is more abrupt, they appearing only as supports of the spires. 



The typical species of the genus [A. reticidaris) is known to exist in 

 most of the geological formations from the Clinton group to the Chemung 



