102 



THE GEOLOGIST. 



Genus Steophomena. Rafinesque. 1820. 



This palaeozoic genus appears to have been as much restricted in its vertical 

 range as was Orfhis, for but a single species is known to me from the Scottish 

 Carboniferous strata, and no well authenticated example of the genus appears 

 to have been observed in any of the subsequent periods. The shells of which 

 this group is composed vary considerably m shape and character, being 

 generally semi-cu-cular, with a long straight hinge-line, the ventral valve being 

 either convex or concave, while the dorsal one usually follows the curves of the 

 other. 



XXIY. — Steophomena ehomboidalis, Wahl., var. analoga, Phillips. PI. i., 



figs. 26-33. 



Ammifes rhomboidalis, Wallenberg, Acta. Soc. Tips., vol. iii., p. 65, No. 7, 1821. 

 = Produda depressa, Sowerby, Min. Con., tab. 459, 1823 ; Producta anal- 

 oga, Phillips' Geol. of York, vol. ii., p. 215, pi. vii., fig. 10, 1836. etc. 



Of this species there are two weU marked varieties, the first or typical 

 one varies somewhat in shape, but is more often semicircular, with a very 

 long hinge-line. The ventral valve is geniculated, or, in other words but slightly 

 convex or flattened up to a certain distance, and up to a certain age, when 

 the valve becomes suddenly deflected downwards at almost right angles. 

 The margin is undulated ; concave near the cardinal angles, it afterwards bulges 

 out to form in front another slight inward or outward curve. On the flattened 

 portion of the vahe there exists a variable number of slightly undulating and 

 irregular concentric wrinkles which turn outwardly towards the cardinal angles, 

 and thus foUow the marginal curves. The entire surface is also covered with 

 numerous thread-like radiating striae, and a small circular foramen is generally 

 observable in the young, or up to a certain age, when it becomes obliterated 

 or cicatriced in the adult. The dorsal valve usually follows the curves of the 

 opposite one, and is similarly wrinkled and striated. The area in both is 

 narrow and sub-marginal, with a small fissure in the ventral one, partially covered 

 by a pseudo-deltidium. In the interior of the ventral valve two diverging 

 teeth articulate with corresponding sockets in the opposite valve. The muscu- 

 lar impressions in this valve are margined by a semicircular ridge, continued 

 from the base of the teeth, and curving on either side so as to produce a saucer- 

 shaped depression ; the occlusor leaves a scar on either side, close to a small 

 median ridge, the devaricator filling on either side the anterior portion of the 

 cavity ; the ventral, adjuster, and pedicle muscles do not appear to have pro- 

 duced any very definite scars, but it is highly probable that an attachment for 

 these muscles existed in the posterior portion of the saucer-shaped depression 

 above _ described, from the fact that a small circular peduncular foramen is also 

 sometimes observable at a short distance from the extremity of the beak, and 

 which denotes that a pedicle muscle must have existed, although the foramen 

 became closed as soon as the animal found it could dispense with the moorings 

 required during the early stages of its development. In the interior of the 

 dorsal valve the cardinal process is divided into two lobes, and not connate ^dth 

 the diverging socket-ridges. Prom the base of this a slight median ridge runs 

 down and separates the two pairs of occlusor scars, which are bordered by pro- 

 minent ridges. The vascidar impressions consist of large primary vessels, which 

 run at once direct to a short distance from the frontal margin, when they be- 

 come reflected on cither side to siUTOund the ovarian spaces. 



The second variety, or Lcpt. distorta of J. de C. Sowerby (Min. Con., tab. 

 615, fig. 3) is more properly speaking a malformation of 'the Stroph. rhom- 

 boidalis, var. analona, and in which the" dorsal valve becomes convex instead of 

 concave; but ;dl the other characters arc similar to those of the typical shape. 



