362 . Systematic Paleontology 



there is a well-developed bidentate cardinal process, or the two parts may 

 be coalesced into a simple process. 



LIncinulus vellicatus (Hall) 

 Plate LXIV, Figs. 7-10 



Rhynchonella vellicata Hall, 1857, Tenth Ann. Rept. N. Y. State Cab. Nat. 



Hist., pp. 69, 71, figs. 2, 3. 

 Rhynchonella vellicata Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 230, pi. 



xxxiil, figs, la-lp, 1861. 

 Vncinulus vellicata Hall and Clarke, 1893, ibidem, vol. viil, pt. ii, p. 199. 



Description.—" Shell varying from transversely oval to subtriangular : 

 ventral valve depressed convex; beak somewhat prominent, depressed, 

 closely incurved over the opposite : dorsal valve more gibbous ; beak in- 

 curved, not prominent. Surface marked by twenty-four to thirty-six 

 plications, six to eight of which are elevated in front of the dorsal valve 

 so as to form a rather distinct mesial prominence, rarely extending beyond 

 the middle of the valve. On the ventral valve, five to seven of the plica- 

 tions are depressed, towards the front, into a more or less distinctly defined 

 sinus, and prolonged, forming a mesial projection, which is more or less 

 elevated in the front of the other valve. Near the junction of the valves 

 in front, very fine closely arranged lines of growth are visible." Hall, 

 1857. 



As this is not one of the common rhynchonelloids which as a rule are 

 inconstant in their characters, it is not possible to ])oint out the variations 

 between the southern and northern stocks. On the other liand, U. velli- 

 catus is closely related to U. abruptus, a fact that did not escape Hall, 

 for he states that " there are, indeed, some forms which it is difficult to 

 distinguish " from one another. The difference, however, that will dis- 

 tinguish U. ahruptus from U. vellicatus is that the former is more trans- 

 verse, the plications on each side of the dorsal fold are more sharply 

 angular, and their crests are at one side of the center, nearer the median 

 fold. Then, too, U. ahruptus attains a larger growth. It must be ad- 

 mitted that these differences are not of great importance, and it may be 

 that larger collections will show that both repre,-^ent one species or that 



