130 
PALAEONTOLOGY OF NEW YORK. 
giving expresssion to this class of fossils as they occur in these rocks. Since, 
however, this is the ordinary if not universal condition of some of the species, 
we have no other means of illustrating the fauna, nor the student of designating 
the species than from the casts; and by those alone are the formations 
sometimes to be distinguished.* 
The following species of Loxonema, illustrated upon plate 28, appear to 
be quite distinct from those described in the preceding pages. 
Loxonema rectistriatum, n. sp. 
PLATE XXVIII, FIG. 9. 
Shell elongate terete. Volutions probably twelve or more in number, moder¬ 
ately convex, very gradually increasing in size, the last one being scarcely 
more ventricose than the preceding; each volution is distinctly contracted 
a little below the close suture, and then expanding gives the greatest 
convexity near the lower third. Suture-line close. Aperture ovate, with 
the columella extending below. 
Surface marked by slender, gently curving longitudinal strise, which bend 
backward from the suture to the bottom of the constriction, and then 
continue to the base of the volution—those of the last one curving gently 
forward to the columellar lip. The spaces between the striae are from 
once and a half to twice the width of the ridges. 
This species may be distinguished from any other described in this volume 
by the finer longitudinal striae, which are scarcely curved on the body of the 
volution, and also by the constriction of each volution just below the suture¬ 
line. The striae are stronger on the upper volutions, gradually becoming finer 
and less prominent on the lower ones, though continuing distinct throughout. 
The specimen figured, which preserves about eight of the lower volutions, 
measures a little more than one inch in length. 
* Where the specimens occur in the condition of casts only, the difficulties of determining the genera 
Loxonema and Mukchisonia are often insurmountable ; and among Platyceras, Cyclonema, Pueurotomaria 
and Callonkma, the forms are so nearly alike that the casts offer few characters for generic determination. 
