118 SYNOPSIS OF THE CHARACTERS OF THE 



Leptagonia nodulosa. Phil. sp. 



Leptsena nodulosa. Phil. Pal. Fos. 



Sp. Ch. — Shell semicircular; concentric rugae disconnected, very irregular, crossed by extremely fine, 

 flexuous striae ; deflected front, undulated, so as to make the angle which separates it from the rostral portion 

 somewhat nodular ; the front is not flattened in the middle. 



This is a larger, wider, and more irregular species than the L . analoga ; the concentric wrinkles on the flat, 

 rostral portion are very short and irregular, and do not extend the whole width of the shell; in the L. ana- 

 loga they are decussated by rather coarse, straight, radiating strije, while in the present species the radiating 

 striae are remarkably fine and waving. The deflected front is so much undulated, that it appears almost nodu- 

 lar, whence the specific name; it wants also the mesial depression of that species. It is difficult to say whether 

 this is really distinct from the common L. analoga, for instance, it is usually found much larger than that spe- 

 cies, but that might only shew it to be an adult individual; then, again, the irregularity of the deflected margin 

 and concentric rugae are just the characters we would expect to arise from extreme age; the radiating striae I 

 have found to vary very much in thickness, &c., according to the preservation of the specimens ; yet there 

 appears to be a peculiarity of contour, which, together with its being most abundant in the Devonian rocks, 

 renders it desirable to retain Professor Phillips' specific name. Length one inch three lines, width two inches 

 nine lines. 



Leptagonia plicatilis. Sow. sp. 



Productus plicatilis. Sow. Min. Con. — Producta plicatilis. Phil. GeoL York. 



Sp. Ch Rostral portion convex, with a deep, narrow, mesial hollow, and sixteen or seventeen small, 



rounded, concentric wrinkles, irregularly splnose ; a row of three or four large spines on each side the beak ; 

 deflected front smooth. 



This is obviously an aberrant species, leading to the ordinary Productas by its convex, rostral portion, and 

 also by its having a mesial hollow and large spines. In well preserved specimens the front is perfectly smooth, 

 even the fine, radiating striae being there obsolete. Length seven lines, width eleven lines. 



Leptagonia rugosa. Dal. sp. 



Leptsena rugosa. Dai. 



Sp. Ch. — Subrhomboidal, front straight, deflexed margin short; flat portion of the valves with fine, 

 regular, rounded, concentric undulations, crossed by coarse, direct, equal, radiating striae. 



This species is very rare in the mountain limestone, and, when it does occur, is often confounded with the 

 P. analoga, Phil., from which it differs entirely in its striation, and in the great regularity of the transverse 

 wrinkles. 



Leptjena. Dal. (Restricted). 



Gen. Ch. — Semicircular, thin, scale-like; finely striated longitudinally ; dorsal valve convex, ventral valve 

 nearly equally concave ; hinge-line as long as the shell is wide, usually armed with spines ; cardinal area dis- 

 tinct, flat, until parallel sides, having a central, triangular, nearly closed foramen. 



The genus, as here restricted, includes all those elegant, nearly flat, striated, scale-like shells, so abundant 

 in the Silurian rocks, but rare in the carboniferous series. They are distinguished from Orthis by their nar- 

 row, cardinal area, with parallel sides, the area of Orthis being triangular, or rhomboidal, when distinctly com- 

 mon to the two valves ; the hinge-line is never less than the width of the shell, and frequently armed with 

 comparatively large spines, in which they differ from the true Orthida, which are never spinose. 



