476 



THE GEOLOGIST. 



V. — Athtris plano-sulcata. Phillips sp. PI. xii, figs. 10, 11. 



Spirifera 'plano-sulcata, PhiUips, Geology of Yorkshire, vol. ii., p. 220, pi. x., 

 fig. 15, 1836; and Dav. Mon. Garb., p. 80,pl.xvi., figs. 2-13,15. 



The shape of this shell is more often orbicular, the valves being equally deep 

 and evenly convex, without sinus or fold, or with a slight mesial depression 

 towards the front in one or both valves. The beak is small and incurved, with 

 a minute foramen placed close to the umbone of the opposite valve. When 

 perfect, the surfaces of both valves are ornamented at intervals of less than a 

 Hne, with numerous concentric semi-circular lameUiform expansions, each plate 

 being flat and longitudinally striated. It is, however, rare to obtain specimens 

 in which the plates are in place, as they generally remain in the matrix from 

 whence the shell is detached. The interior arrangements are similar to those 

 of A. amhigua. 



A. plano-sulcata does not appear to be a very common shell in Scotland ; it 

 occurs at three hundred and forty-three fathoms below the "Ell coal," and near 

 Lesmahago, in Lanarkshire. In Stirlingshire it occurs at Craigenglen. In Ren- 

 frewsliire, at Arden quarry, Thornliebank, Barrhead, etc. ; in Ayrshire, at 

 Eoughwood and West Broadstone, near Beith. It has also been found in West 

 Lothian. 



VI. — Athyeis Royssii. L'EveiUe. PI. xii, fig. 12. 



Spirifer de Roifssii, L'EveiUe, Memoirs de la Societe Geologique de Prance, vol. 

 ii., p. 39, pi. ii., figs. 18-20, 1835 ; and Athyris Uoyssii, Dav. Mon. Garb., 

 p. 81, pi. xviii., figs. 1-11. 



This species is generally transversely oval and sub-globose, with equally deep 

 and uniformly convex valves up to a certain age ; after which a mesial fold of 

 greater or lesser elevation is gradually formed in the dorsal valve, and to which 

 corresponds a sinus in the opposite one. The frontal margin is, therefore, 

 either nearly straight, or presenting a greater or lesser curve. The beak is 

 incurved and truncated by a small circular foramen, which is contiguous to the 

 umbone of the dorsal valve. Externally the entire surface is regularly covered 

 with numerous concentric scaly ridges, from each of which radiate closely set 

 fringes of elongated somewhat flattened spines ; and, indeed, so closely packed 

 are the spiny ridges, that in the perfect shell no portion of the valve could be 

 perceived. Specimens are not, however, to be obtained with their spiny invest- 

 ment from the hard limestone matrix, but from the decomposing shales exam- 

 ples may sometimes be picked up in which portions of the spines are still pre- 

 served. The interior arrangements are similar to those of the two species 

 already described. 



Atliyris Royssii occurs in the shales of Coalburn and Brockley, near Lesma- 

 hago, and at West Broadstone, near Beith. 



SuB-GENTJS Retzia. King. 1850. 



The species which compose tliis sub-genus are Terebratula-shaped, with a 

 perforated, or punctured shell-structure, and by this character they appear to be 

 chiefly distinguished from Athyris. Interiorly they possess spiral processes for 

 the support of the oral arms, with their extremities directed outwards ; but I 

 have never yet succeeded in procui'ing a specimen wherein the details could be 

 satisfactorily developed. I am acquainted with but a single species from the 

 Scottish ciu'bonifcrous strata. 



