144 
PALEONTOLOGY OF NEW YORK. 
when removed, leave the surface with only regularly concentric growth-lines 
marked by papillae 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 Nucleospiridje, 
viz.; Retzia, Meristina, Eumetria and Trematospira, but the distinctive 
features of the brachidium bring it into closer association with Anoplotheca 
and Cgelospira, 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 lamellaB 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 or Anoplotheca, nor is its extremity bifurcated to em¬ 
brace the low median septum of the valve. The stem usually makes a large 
