GEOLOGY AND MINERALOGY. 161 



§ 2. Shell concavo-convex, genei-ally depressed ; widest at hinge-line ; 

 narrow area, with deltidium or pseudo-deltidium ; no spines. Beak of re- 

 ceiving valve often perforated by a minute foramen, 



Strophomena (including Leptsna, Also Leptagonia, McCoy ; Peridiolithus^ 

 Hiipsch ; Enteteles, Fischer ; Plectamhonites and Gonamhonites, in part, 

 Pander; 8trophodonta, Hall). Many palseontologists — McCoy, d'Orbigny, 

 Pictet, &c. — separate Strophomena and Leptccna as distinct genera; but the 

 opening in the beak of the former is often closed, and the curvature of the 

 valves appeal's to be a character of uncertain value. Some species of Lep- 

 tsena also, have a small foramen at the beak of the receiving valve. 



Appendix : — Davidsonia, Bouchard , Tropidoleptus, Hall. 



§ 3. Shell concavo-convex, with tubular spines. 



Spines placed irregularly ; area very narrow or obscure : — Productus, Silu- 

 rian to Permian. 



Spines placed irregularly ; area broad, with pseudo-deltidium : — Stropha- 

 losia, King. (^Aulostegeit, Helmerson ; Orthothrix, Geinitz. Including also 

 King's Leptcenalosia) . McCoy makes this a sub-genus of his Leptsena. De- 

 vonian to Triassic. 



Spines on hinge-margin of receiving valve only : Gftonetes. Silurian to 

 Permian. 

 Spiriferidce : — Shell with internal spiral processes pointing outwards. 



§ 1. Hinge-line moi"e or less straight, with well-developed ai'ea but no 

 deltidium. 



Shell impunctate ; opening triangular, slightly trenching on the entering 

 valve : — Spirifer (Delthyris, Dalman ; Ghoristetes, Fisher ? Trigonotnta> 

 Koenig). Silurian to Triassic. 



Shell punctate ; opening triangular, bordered, and confined to receiving 

 valve : — Spiriferina, d'Orb. (Includes many so-called spiriferg.) Jurassic — 

 apparently confined to the Lias beds. 



Shell punctate or impunctate, pyramidal ; opening small, generally obscured 

 or altogether closed by the large pseudo-deltidium. Area very large: — Cyrtia, 

 Davidson proposes to place the punctate species imder Spiriferina. 



§ 2. Hinge-line curved. Area generally wanting, but a deltidium usually 

 present. Whole shell more or less terebratuhform in its aspect. ' 



Shell impunctate. Small opening, generally closed, at extremity of beak. 

 No deltidium. Area indistinct, or wanting: — Athyris CSpirigera d'Orb.; 

 Gleiothyris, King), Silurian to Triassic. 



Appendix : — Merista, Suess ; Meganteris (?) Suess, 



Shell impunctate, and much like Athyris, but with a deltidium of two united 

 pieces, although this is often concealed : — Spirigerina (Atrypa, Dalman ; 

 Gleiothyris, Phillips ; Hipparionyx, in part, Vanuxem), Silurian, Devonian. 



Shell punctate, a round foramen at beak, and small triangular area : — Retzia. 

 Silurian to Carboniferous (or Triassic 1). Externally, this genus cannot be 

 distinguished from many Terebratulas, 



Shell impunctate, with long incurved beak. No area, but large concave 

 deltidium : — Uncites ( Qypidia, in part, Dalman), Devonian, 

 VOL. III. L 



