144 PALEONTOLOGY OF NEW YORK. 



when removed, leave the surface with only regularly concentric growth-lines 

 marked by papillse which are the bases of the spinules. 

 Type, Spirifera ventricosa, Hall. Lower Helderberg group. 



Observations. Nucleospira is a well defined and very compact genus. 

 Though not largely represented in species, it has a considerable vertical range, 

 appearing in the middle Silurian and disappearing probably in the middle or 

 upper Devonian. • The genus is allied in some general respects to the genera 

 which Mr. Davidson associated with it, under his sub-family NucLEospiniDjs, 

 viz. : Retzia, Mekistina, Eumetria and Trematospira, but the distinctive 

 features of the brachidium bring it into closer association with Anoplotheca 

 and CcELOSPiRA, notwithstanding the great differences in exterior. The spinul- 

 ous surface, which appears to be common to all the species, is suggestive of 

 Parazyga and some forms of Athyris. The peculiar structure of the delthy- 

 rial covering, resulting from a coalescence of the deltidial plates, is not unlike 

 that of Eumetria and Trematospira, and the same extreme of modification is 

 reached in Parazyga Deweyi. The imperforate hinge-plate is extravagant in its 

 elevation and peculiar in form. Hitherto the character of the loop has not 

 been accurately determined, and yet this structure is very simple ; a union of 

 the lateral branches at a low angle and the continuation of an undivided 

 straight stem across the interior cavity. The explanation of the various 

 imperfect determinations of this part which have been given by different au- 

 thors, is to be found in the fact that the long, slender crura, the broad umbonal 

 blades of the primary lamellae and the lateral branches of the loop almost, and 

 sometimes actually meet. Any detachment of the interior part of the shell, 

 either intentional or accidental, is very likely to pass through this point of 

 convergence and remove from the seven processes there approximating the 

 simple continuation of the loop. This stem of the loop is continued beyond 

 the bases of the spiral cones, and may reach the surface of the pedicle-valve, 

 but it does not articulate in a slotted ridge in the similarly constructed loop 

 of the genus Bifida oi' Anoplotheca, nor is its extremity bifurcated to em- 

 brace the low median septum of the valve. The stem usually makes a large 



