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PRODUCTA costata. 
TAB. DLX.—/fg. I. 
Spec. Cuar. Transversely oblong, with an angu- 
lar depression in the middle, costated ; coste 
few, broad, decussated at their upper parts, 
compressed upon the deflected front; each 
side furnished with two or three spines and a 
small lobe. 
Srrone sulci divide the surface of this shell into 18 
broad ribs, which are crossed by concentric furrows upon 
the upper part of the convex valve: at each side is 
formed a small lobe, upon which are two or three spines : 
the front is much produced and deflected. 
For the use of this strongly marked Producta we are 
indebted to Mrs. Murchison. It was found near Glas- 
gow; and seems to have been imbedded in a shaley 
Limestone. 
— 
PRODUCTA calva. 
TAB. DLX.—/figs. 2 to 6. 
Spec. Cuar. Nearly square, smooth, flat or con- 
cave in the middle; beak much incurved; one 
row of spines on each side the hinge-line. 
Var. «. Small, flat or slightly concave in the 
middle. jig. 2. 
Var. 8. Twice the size of var. «, with a canal 
along the middle. figs. 3—6. 
Exrernauty this species is remarkably smooth; but, 
like many others, it is scabrous within, or beset with 
