134 



BRITISH SILURIAN BRACHIOPODA. 



Ventral valve almost flat, or very slightly convex near the beak, w^ith a rather deep and 

 sharply defined sinus, margined on either side by a prominent ridge, vi^hile the lateral 

 portions of the valve are gently concave, and vertically turned up at their extremity ; lateral 

 margins undulating ; front margin abruptly raised, straight along the middle ; surface of 

 both valves ornamented with numerous small bifurcating ridges or ribs, of which every 

 three or four are clustered together, and separated by more deeply indented grooves. 

 The shell is also closely crossed by numerous, equidistant, small, concentric, slightly 

 raised lines. Interiorly the spires are vertical, closely adpressed, and directed towards 

 the disk of the valve. 



Length 8, width 9, depth 5 lines. 

 Obs. With the exception of Dr. Lindstrom, palaeontologists have united under a 

 single species the Terebratula marginalis of Dalman and T. imbricata of J. de C. Sowerby ; 

 still, as pointed out by Lindstrom, they would appear to be distinguished by several 

 well-marked characters, which may be briefly enumerated. In Atrypa marginalis the beak 

 is straight and prominent, though small, with a circular foramen under its slightly 

 incurved extremity, distinctly separated from the hinge-line by a well-defined deltidium. 

 In A. imbricata, on the contrary, the beak is so much incurved as to almost touch the 

 umbone of the opposite valve ; consequently no deltidium is observable ; the extremity of 

 the beak is pierced by a small foramen. This last-named shell has also both its valves 

 more convex than those of A. marginalis ; and the external sculpture, or rather the con- 

 centric ridges, are very difierent in the two shells. In A. imbricata they are close, 

 numerous, small, and but little produced; while in A. imbricata they are coarse and scale- 

 hke : but a glance at the respective enlarged figures of both forms, given in Pi. XV, v^ill 

 explain these differences much better than words can. Atrypa marginalis was well 

 described by Dalman in 1827;^ and his figures, although not well executed, show the 

 l)rincipal characters of the species. Dr. Lindstrom observes, moreover, in his valuable 

 memoir on Swedish Silurian Brachiopoda, that A. marginalis differs from A. imbricata by 

 its more flattened and expanded shape, and that the typical form is in some localities 

 represented by a variety with few and coarser ribs, bifurcating several times. Although 

 in the * Silurian System' Sowerby described and figured both shells as separate species, 

 in the ' Tabular List' appended to the same work he considers the two as synonyms of a 



1 " T. testa radiatim multistriata, margine compresso apiceque subreflexo ; valva majore dorso canali- 

 culata ; rostro prominente recto, foramine parvo apiciali. 



Locus: Gottlandia, in Klinteberg ; Mus. Dom. Hisioger. 

 Specitnina majora longit. 18 lat. 20 m.ni. 



Testa undique confertira longitudinaliter striata vel siibsulcata, costis 40 — 50 ; videlicet nonnullis 

 apicem versus furcatis ; de caetero inscqualis, margine compresso, ssepius sub-explanato, interdum reflexo. 

 — Valva minor jugo dorsali parum elevato, striate, margine apicali sinuato, L. subemargiuato. — Valva major 

 canali dorsali profundo striato, apice sinuato ; rostrum acutiusculum, proniinens, rectum, foramine parvo 

 Hubapicali. — Valvulse miuoris prima basis subrostro occultata, nate baud raanifesta." 



The figure is, however, much too finely striated. 



