STROPHOMENIDtE. 283 



Strophomidna EHOMBOiDALis, Lindstrom. Gotlands Brach., p. 371, 1860. 



— — Haswell. Sil. Form. Pentland Hills, p. 33, pi. iii, fig. 



3, 1865. 



— — Bav. Trans. Geol. Soc. Glasgow, Pal. Ser., vol. i, p. 



16, pi. ii, figs. 17—18, 1868. 



Spec. Char. Shell more or less transversely semicircular or sub-quadrate ; valves 

 geniculated; hinge-line straight, and as long as the greatest width of the shell, with 

 rounded cardinal angles, which are at times prolonged in the shape of expanded wings. 

 The ventral valve is slightly convex at the beak, and away from this it becomes flattened 

 to a certain distance ; also with age, when the valve is suddenly bent downwards at almost 

 right angles. The frontal margin is undulated, concave near the cardinal angles ; it 

 afterwards bulges out laterally, to form a slight outward curve in front. On the flattened 

 portion of the disc there exists a variable number of slightly undulating and occasionally 

 intercepted concentric wrinkles, which turn outwardly towards the cardinal angles, and 

 thus follow the marginal curves. The entire surface is also covered with numerous 

 radiating thread-like striie ; and a small circular foramen is generally observable, up to a 

 certain age, close to the extremity of the beak, but it becomes obliterated or cicatrized in 

 the adult. The dorsal valve is concave, usually following the curves of the opposite valve, 

 and is similai'ly wrinkled and striated. In the interior of the ventral valve two diverging 

 teeth articulate with corresponding sockets in the opposite valve. The muscular 

 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 

 adductor or occlusor leaves a scar on either side, close to a small median ridge, the 

 cardinal or divaricator muscles filling on either side the anterior portion of the cavity ; the 

 ventral adjustor and pedicle muscles do not appear to have produced any very definite 

 impressions, but it is highly probable that an attachment for these muscles existed in the 

 posterior portion of the saucer-shaped depression above described, on the account of the 

 fact that a small circular peduncular foramen is also abservable at a small distance from 

 the extremity of the beak ; and this denotes that a pedicle muscle must have existed, 

 although the foramen became closed as soon as the animal found that 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 with the 

 socket-ridges. l\'om the base of this a slight median ridge runs down and separates the 

 two pairs of adductor or occlusor scars, which are bordered by prominent ridges. The 

 vascular impressions consist of large primary vessels, which run at once direct from near 

 the centre of the valve to a short distance of the frontal margin ; some of the vessels 

 bifurcate several times. Dimensions very variable ; two specimens measured — 

 Length 14, width 20, depth 4 lines. 

 15, „ 4.2, „ U „ 



