PRODUCTUS. 169 



sinus from the level of the hinge-line to the margin, bearing a longitudinal row 

 of four or five very large spines. Cast ornamented with very numerous, 

 reticulating, longitudinal threads, which show a tendency to group themselves 

 into more or less incipient ribs. Wings with a few coarse tubercles (spines) near 

 the corners. 



Dorsal valve transverse, geniculated, with a minute umbo ; having (as seen 

 from within) a large elevated triangular ridge down the centre, and seven or eight 

 smaller broken ridges on each side, which increase in size as they cross the flat 

 part of the shell, and extend to the margins ; the whole nodulated by rugose 

 transverse undulations, which are minute near the umbo, but soon become very 

 coarse. Ears convex, prominent, marked only with transverse ridges. 



Size. — About 35 mm. long and wide. 



Localities. — Croyde, Braunton, Saunton, Top Orchard, Kingdon's Shirwell, 

 Ashford Strand, Upcott Arch Quarry, Wrafton Lane, Rock Inn Quarry, Pouch 

 Bridge. It is very abundant, generally gregarious ; but it seems local, probably 

 being confined to limited beds, which in the disturbed condition of the Pilton 

 Series it is not easy definitely to trace. 



Remarks. — The external surface of this shell does not appear to be known. 

 The ventral valve occurs in a state that shows a coarse ramifying fibrous texture, 

 which has the appearance of having originally been covered by an additional layer 

 of shell. This valve is remarkable for the incipient central keel in the midst of 

 a rather definite shallow groove or sinus, which bears a row of very large and 

 probably long, cylindrical, or slightly clavate spines. There are also a few coarse 

 spines on the ears. The ribs are rounded and more or less indistinct, and there 

 seem no signs of spines upon them. 



The dorsal valve is so dissimilar from the ventral valve in shape and markings 

 that it was regarded by my friend Mr. Townshend Hall as a distinct species ; but 

 Sowerby's type, which is in the Woodwardian Museum, preserves both valves in 

 contact, as pointed out by M'Coy, and thus proves their true relationship. On 

 account of its geniculate form it is always its inner face which is exposed, and I 

 have therefore described it from that point of view. If it were seen from the 

 outside its characters would probably be almost exactly reversed. 



2. Phoductus PRiELONGUS, Sowerby ? var. simpliciok, n. var. Plate XX, figs. 14, 



14 a, 15, 15 a. 



18G5. Phoductus longispinus ?, Davidson. Brit. Foss. Bracb., vol. iii, p. 103, 



pi. xx, fig. 7. 

 189G. — — Wlridborne. Proc. Geol. Assoc, vol. xiv, p. 376. 



Y 



