1 16 SYNOPSIS OF THE CHARACTERS OF THE 



Producta sulcata. Sou-. 



Productus sulcatiis. Sow. Min. Con. — Producta sulcata. Pkil. Geol. York. 



Sp. Cli. — Semicylindrical, front flattened, with parallel sides; mesial liollow small, shallow; longitudinal 

 ribs very thick, rounded, at intervals spinous, decussated near the beaks by slight, concentric wrinkles, a row of 

 large spines on each ear. 



This species is sometimes likely to be confounded with some varieties of the P. antiquata, but differs in its 

 smaller size, greater proportion of length to width, more strictly parallel sides, and the top of the smaller valve 

 being flat ; the radiating ribs are also much larger and smoother. Professor Phillips unites the Producta coa- 

 tata, Sow., to this species ; but the specimens I have been able to examine of this latter agreed strictly with 

 Mr. Sowerby's figure and description in the Mineral Conchology ; and even fragments may be distinguished by 

 the shape of the longitudinal ridges in each, so that it may, perhaps, be as well to keep the species distinct. I 

 have only seen the Producta custata from the original locality in which they were found by Mrs. Murchison, 

 namely, the Glasgow coal shale, wliile the Producta sulcata is common in many limestone districts in Ireland. 

 Length two inches, width one inch and a half 



Producta tortilis. M'Coy. (PL XX. fig. 14). 



Sp. Ch. — Semicircular, convex ; rostral portion rather depressed; front deflected, rounded; entire sur- 

 face marked with close, fine, rounded, longitudinal strife ; and transverse, interrupted, irregular, waving wrinkles. 



This remarkable species is, perhaps, most nearly allied to the P. Scotica, Sow., and it also bears some re- 

 semblance to the P. f raff aria ; but both those species are distinctly spinosc, while the present shell is altogether 

 without spines: from the P. hemisplierica, Sow., and P. comir/ata, M'Coy, it is distinguithed by the depres- 

 sion of the rostral portion, and by the irregular, transverse, wrmkling, or crumpled appearance of the whole sur- 

 face. Length one inch, two lines, width one inch six lines. 



Leptagonia. M' Coy. 



Gen. Ch. — Shell transverse ; rostral portion flat, concentrically wrinkled ; front smooth, abruptly deflected . 

 at right angles to the flat, rostral portion; cardinal area flat, with a large (obtect) triangular foramen. 



The remarkable fossils which I separate under this name have all a pecuHar contour, from the distinct 

 angle which separates the flat, wrinkled, rostral part of the shell, from the smooth, deflected front. Von Buch 

 places the shells which enter into the present genus among the Delthyrida or Spirifers, a position which almost 

 every other author denies ; most writers agreeing that they should be placed among the Producta, with which 

 they certainly agree in all their important characters, excepting the cardinal area and foramen. It is by this 

 group that the Athyridm or Productas pass into the DelthyridcB or Spirifers, uniting in itself the characters 

 of the two groups. The generic characters above given are merely those external ones which can be easily found in 

 the worst preserved specimens, and will distinguish even fragments from the other ge- 

 nera of the Productas. The internal structure difiers very much from that of the other 

 groups ; the muscular impressions are four in each valve. In the dorsal valve the prin- 

 cipal pair of adductor muscles (a a, fig. II) arise each from a large, rounded, radiatingly 

 sulcated cicatrix, placed about one-third their diameter from the beak, and divided by 

 a slight, shelly septum (c) ; the cicatrices of the lesser adductors (b b) are much smaller 

 than the others, pear-shaped, the anterior end being rounded, the posterior attenuated, 

 smooth ; they take their origin interiorly, and a little anterior to that of the great pair. In this valve we also 

 observe two large cardinal teeth (d d), perfectly resembling those of Orthis. In the ventral valve the attach- 



