TRENTON LlMESTONi:. 105 



The surfaces of both these specimens present the essential characters of the L. alUrnata, 

 and the beak is always perforated. This is shown in the enlarged view of the hinge in 

 fig-. 1 a, whieli at the same time shows the foramen not quite closed. 



This nasutc feature is often connected with a single elevated ridge, or more prominent 

 stria, much longer than the others, and extending from the base to the beak of the shell. 

 Tliis appears very distinctly in the two preceding specimens ; but to show that this character 

 is common to the species, the specimen fig. 1 d is represented, showing a prominent line 

 down the centre, while the shell is totally destitute of the other characters of S. nasula, being 

 scarcely convex and very regularly somioval. 

 Figs. 1 c,f,g&h. These figures represent a series of the younger shells of this species, as they occur 

 in the compact limestone of Middleville, Little Falls, Trenton Falls, and other places. The 

 strinj often alternate irregularly, and sometimes they are of nearly equal size. This variation 

 ol'tcn arises in part from exfoliation of the sliell ; but if a number of specimens are examined, 

 there will be found a gradation in the strite, and some of them exliibit the typical characters 

 of the species. 



These figures present also a gradation in form from the perfectly semioval to the elongated 

 or nasute front, showing very conclusively that this character cannot be relied upon in the 

 .distinction of species.* ~ 



Position and locality. This is one of the species, which, (omiiu'iicing its existence prior 

 to, or at the epoch of the Trenton limestone, continues in great ruinibers throughout that 

 rock, and, though not appearing in the Ulica slate, reappears in the Hudson-river group 

 in immense numbers, several thin strata in the upper part of this group being composed 

 almost entirely of the shells of this species. 



The forms 1 e,f, g Si, h, of Plate XXXI. A., are the most common and abundant in the 

 Trenton limestone, and some of these are met with in almost every locality : the other 

 forms are more rare. 



It is aluindant at Middleville, Little Falls, Jacksonburgh, Herkimer, Trenton Falls, 

 Sugar River, Turin, Lowville, Watertown, Plattshurgh, Glen's Falls, &c. 



The same species, in forms like 1 g, h, i, k, Plate XXXI., and others, are abinidant at 

 Maysville ( Ky.), Cincinnati and Oxford (Ohio), Madison ( Indiana), and in Tennessee. 



(State Collection.) 



• 1 liave been induced to dwell so loiii< upon tliis s|jc'cie.s, I'rom the fact of its variable form, and other characters 

 which have been the source of much annoyajice to amateurs, and no little trouble to myself, since I have it from 

 diflereut persons and localities, under at least six or eight dilierent names. 



[Pal-eontology.] 14 



