INVERTEBRATE PALEONTOLOGY. 9 



Tl)t> existing genus Glottidia, Dall, is likewise closely related to Lingula, 

 but differs in having two internal projecting lamina', diverging and extending 

 forward from the beak of the ventral valve about one-third of the length of 

 the shell. 



The genus Lingula was introduced at a very early period ; at least, we 

 find species that are in no way distinguishable from it by any external 

 characters, in some of the oldest Silurian rocks. Its shells are also found 

 through all the subsequent formations, and several species are known to 

 inhabit our existing seas. It seems to have attained its maximum develop- 

 ment during the Silurian age. In rocks of all ages its shells present very 

 nearly the same dark, shining appearance, and have been found to possess 

 the same more or less phosphatic composition. 



The existing species are found on the coasts of the Sandwich and 

 Philippine Islands. They inhabit shallow water, being generally found at 

 low tide, with their long peduncle deeply penetrating the sand or mud. 

 The species often mentioned in lists, from North and South Carolina and 

 California, are said all to possess the internal character of the genus Glottidia. 



Lingula 11 i t i cl a , M. & H. 



Plate 28, figs. IS, a, b. 

 Lingula nitida, Mock and Haydeii (1861), Proceed. Acad. Nat. Sci. Philad., 443. 



Shell small, moderately convex, extremely thin, narrow-subelliptic in 

 outline, the greatest breadth being near the middle and generally less than 

 half the length; front very narrowly rounded, sides forming very slightly 

 convex curves; beaks rather obtusely pointed, that of the ventral valve being 

 a little more prominent than the other; valves nearly equally convex, the 

 greatest convexity being along the middle ; surface polished, and only marked 

 by very fine lines of growth, which are obsolete on the more convex parts 

 of the valves, but become moderately distinct on each side. 



Length, 0.33 inch; breadth, 0.16 inch; convexity of the two valves 

 about 0.09 inch. 



This shell may be readily distinguished from L. subspatulaia, H. & M. 

 (Mem. Am. Acad. Arts and Sci. Boston, V, 380, pi. 1, figs. 2, a, b), the only 

 other species known from these rocks, by its uniformly smaller size, pro- 

 portionally narrower and more convex form, as well as by its narrowly rounded 

 2 H 



