CARBONIFEROUS FOSSILS OF IRELAND. 73 



cated ; body of the shell with narrow, obtuse, radiating ribs, closely ornamented witli compressed, transverse 

 tubercles. 



This is one of the most singularly shaped shells with which I am acquainted, all the usual proportions of 

 the genus being reversed ; it resembles the blade of a razor in form, the anterior side resembling the broad, 

 truncated end ; the hinge-line at the posterior side is about one-third longer than that of the anterior side 

 of the beak; the posterior side Is, however, very narrow, and almost pointed; the posterior slope Is small' 

 strongly striated across, and separated from the body of the shell by a slight ridge ; the anterior space, contrary 

 to the usual rule. Is of great size, concave, and nearly smooth ; It Is very obliquely truncated, so as to form an 

 acute angle with the ventral margin ; It Is separated from the body of the shell by a strong, curved ridge, 

 the body of the shell Is convex, having about thirty-three radiating ribs, closely covered with transverse, com- 

 pressed tubercles, giving a reticulated appearance to the surface. Length five lines, width two Inches. 



Byssoarca obtusa. Phil. sp. 



Cuculliea obtusa. Phil. Geol. York. 



Sp.Ch. — Transversely oblong, gibbous, twice as wide as long; beaks large, tumid; anterior end obtusely 

 rounded ; posterior end oblique, very obtusely rounded ; hinge-line three-fourths the length of the shell ; sur- 

 face smooth, with a few obtuse, transverse undulations on the body of the shell, and an obscure reticulation on 

 the posterior slope ; a shallow sinus in the front margin. 



This species has a peculiarly blunt aspect from the obtuse rounding of all the angles : even that at the 

 extremity of the posterior end, which is acute in almost every species of this and the neighbouring genera, is 

 here very obtusely rounded ; the beak is also large, tumid, and bluntly formed : the front is slightly Inflexcd, 

 which produces a few transverse wrinkles on the otherwise smooth body of the shell ; from this evidence of an 

 attaching byssus, I have removed the shell to the genus Byssoarca. Length ten lines, width one Inch ten lines. 



Byssoarca reticulata. M'Coy. (PI. XII. fig. 9)- 



Sp. Ch. — Transversely oblong, gibbous ; dorsal and ventral margins parallel ; beaks moderate, close to the 

 anterior end ; anterior end very short, rounded ; posterior end obliquely truncated ; posterior slope concave, 

 separated from the body of the shell by a strong diagonal ridge ; entire surface striated longitudinally ; trans- 

 versely decussated by the Imbricating lines of growth, of which those nearest the margin are deep and rugose, 

 those near the beak nearly obliterated ; abdominal sinus large, deep. 



This species may be known from the Area cancellata by the greater transverse diameter, and the fineness 

 of the longitudinal strlK : usually larger than the specimen figured, which was the first obtained. Length ten 

 lines, width one Inch eight lines. 



Byssoarca semicostata. MCoy. (PI. XI. fig. 35). 



Sj). C7«.— Width rather more than twice the length, gibbous ; anterior end rectangular ; posterior end nearly 

 rectangular, concave ; posterior slope with seven or eight coarse, rounded, radiating ribs separated by an obtuse, 

 obhque keel from the body of the shell, wlftch is decussated by very minute, transverse, and longitudinal striae. 



This beautiful shell is rendered conspicuous by the large radiating ribs which are on the posterior slope, 

 while the body of the shell appears nearly smooth, being, however, reticulated by microscopic striae. The valves 

 are distinctly unequal. Diflers from the Nucula radiata, Portk., of the Pomeroy schist. In Its rectangular ends 

 and reticulated body. Length four lines, width nine lines. 



Crenella acutirostris. M'Coy. (PI. XI. fig. 37). 

 Sp. C^.— Subrhomboldal, oblique, length and width equal, slightly convex, smooth ; beaks large, promi- 

 nent, acutely pointed ; anterior side rounded, posterior side obliquely truncated, compressed, posterior ridge 

 strong, rounded. 



