472 CARBONIFEROUS LAMELLTBRANCHIATA. 



is almost straight, forming by far the longest antero-posterior diameter. The 

 umbones are obtuse, swollen, not raised, incurved, and placed almost at the centre 

 of the hinge-line. The rostrum has a broad triangular base, continuous below 

 with the posterior border. 



Interior. — Although two of the figured specimens are casts of the interior they 

 give no details, but apparently were more regularly concave and less angular 

 internally. 



Exterior. — Su?^face when well preserved almost smooth, otherwise covered with 

 fine regular radiating ribs. 



Dimensions. — PI. LIU, fig. 4, from Law, Dairy, measures — 



Antero-posteriorly (estimated) . .12 mm. 



Dorso-ventrally . . . .7 mm. 



From side to side . . . .7 mm. 



Localities. — Scotland : the Lower Limestone series of Law, and Glencourt, 

 Dairy, Ayrshire; Potmetal plantation. Bogie, Kirkaldy. 



Observations. — Mr. R. Btheridge, jun., described a new species of Gonocardium 

 under the name G. decnssatnm, which, I think, may possibly represent the species 

 under discussion, though his figures and description are not at all clear, and the 

 type specimens are not to be found. Mr. J. Gr. Goodchild has kindly searched for 

 them in the Museum of the Geological Survey of Scotland, and cannot trace them. 

 Etheridge's figures (op. supra cit.) show the very strong angular keel. He says, 

 " It is a small species, of a regular horse-hoof form, with an exceedingly strong 

 ridge separating the anterior and posterior sides, amounting almost to a keel. In 

 general form it resembles G. trigonale, Phill., and some depressed varieties of 

 G. hibernicum, Sow., but may be at once distinguished from both these by the 

 shell ornamentation." I would remark that in this species, as in all others of the 

 genus, the ornament depends on the state of preservation and on the layer of 

 shell exposed, and that it is impossible to separate the species of Gonocardium by 

 the ornaments or number of ribs on tlie surface. 



Mr. Etheridge's specimens are stated to have been obtained from the Upper 

 Limestone series of Orchard, near Glasgow, and I regret that I have not been able 

 to study a series of shells from this locality. 



This species would seem to occupy a position midway between G. rostratum 

 and G. inflatum ; more angular than either in the body, the posterior end is not so 

 long as that of the former species, and much less adpressed than in the latter. 



