TRENTON FOSSILS. 



FIG. 193. 



composed of joints of unequal thickness and diameter, the larger ones furnished with side arms; surface of 

 the joints deeply striated in radii, which give a serrated appearance to the edges of the plates. 



Lingula; shells of brachiopod mollusks. Four species of Lingula have been found in the Trenton lime- 

 stone in Vermont. The shell of the Lingula is oblong; compressed; slightly gaping at each end; truncated 

 in front; rather pointed at the umbones; dorsal valve rather shorter^ with a thickened hinge margin, and a 

 raised central ridge inside. 



Fig. 193 represents a specimen of Lingula quadrata (Eichw.), of ordinary size. The shell is equivalve, 

 equilateral, broadly oval, depressed convex; sides nearly straight and parallel, or slightly curved; extremities 

 nearly equal in width, the first broadly rounded, cardinal extremity slightly narrower and 

 somewhat angularly sloped; beak marginal, not prominent; exterior surface of the shell 

 marked by strong concentric strife, and along the middle by distinct longitudinal striae, 

 which are equally visible when the outer shell is exfoliated. A longitudinal depressed 

 line marks the shell from the beak nearly half way to the base. 



This is one of the largest Lingula known. Its usual length is about one inch. Its great 

 size is generally sufficient to distinguish it from all other species of the genus in the Trenton 

 limestone. Other characteristic distinctions are its general elliptical form, its parallel sides, 

 and the longitudinal striaa marking the center of the shell. 



Lingula quadrata occurs in Vermont at McNeil's Point, South Hero, and a short distance northwest 

 of the village of Ferrisburgh. 



Lingula dongata (Hall). This shell is represented in Fig. 194. It is an oblong, oval shell, gradually 

 narrowing towards the beak, sides nearly straight, shell much elevated along the center, which continues to 

 the beak, and is slightly depressed in front; a narrow, depressed line extends along the length of the 

 shell, from the beak, more than halfway to the base; surface marked by fine concentric striaa, without vis- 

 ible longitudinal ones. The shell is j of an inch long, and T 6 F of an inch wide. This is a rare shell, and 

 has been found only at McNeil's Point, in Charlotte. 



Lingula obtusa (Hall.) Fig. 195 represents a very perfect specimen of this species of large size; but Fl - 194 - 

 the radiating striae of the species arc scarcely visible. The shell is broad, ovate, obtuse at the apex 

 and regularly rounded below, depressed in front, but very prominently convex on the umbones; beaks 

 obtuse, prominent, not terminal; margin of the shell a little produced beyond the beaks; surface 

 apparently smooth, but, under a magnifier, exhibiting fine concentric and radiating striaa. The 

 shell is easily distinguished by its ovate form, with very obtuse apex; the beak is prominent and 

 elevated, with a narrow space between it and the margin of the shell. The outline from beak to base is 

 much more arched than in any other species. 



The Lingula obtusa has been collected at McNeil's Point and South Hero. 

 Lingula crassa (Hall.) This shell is broadly ovate, with a subacute beak; one valve is 

 more convex than the other, and somewhat arched; the shell is thick, marked by strong, 

 concentric, elevated lines, without radiating striae. The shell is broad and regularly rounded 

 in front, sloping abruptly to the beak. The thick shell and prominent concentric lines are 

 characteristic. The shell is black and less brilliant than the Lingulce usually are. Single 

 valves and fragments of this species are common in Basin Harbor in Ferrisburgh, and just 

 over the State line at Crown Point, N. Y. 



Crania. The shell of the genus Crania is smooth or radiately striated; the umbo of its dorsal valve is 

 sub-central, marginal, or prominent and cap-like, with an obscure triangular area traversed 

 by a central line. 



Fig. 196 represents the shell of a species of mollusk which is doubtfully referred to Crania. 

 It is the Crania filosa of Hall. The shell is orbicular; one valve more or less convex; apex 

 marginal; surface radiated with numerous fine, elevated, thread-like striaa, which are more 

 or less prominent, depending on exfoliation of the shell; intermediate striaa coming in be- 

 tween the others as they recede from the beak; but the striaa are not bifurcate. Young 

 shells of this species are very convex, being almost conical; but gradually become depressed 

 and expanded as they increase in age and size. 



Fia. 195. 



Lingula obtusa. 



Fio. 196. 



Crania filosa. 



