162 GEOLOGY AND MINERALOGY. 



Rhynconellidce : — Shell impunctate, -with slight internal processes. Hinge-line 

 curved. Area wanting, or very narrow. Spiral arms in living species of 

 Ehynconoella directed downwards and inwards. 



No area. M"o internal septa. Incurved beak, small : — Rhynconella. (Includ- 

 ing Atrypa of d'Orbigny, Pictet, &c. : also d'Orbigny's Hemithyris and Acan- 

 ihothyris ; with Cyclothyris of McCoy, <fec.) Silurian, upwards. The so-caUed 

 Atrypae included here, are taken from the genus Spirigerina — comprising the 

 forms without internal calcareous processes or perforation at beak. 



Wo area. A septum in each valve : — Gamerophoria, King. Carboniferous, 

 Permian. Perhaps better placed as the type of a sub-group under Pentamerus. 



Wo true area. Receiving valve divided by internal septa into thi'ee cham- 

 bers. A large incurved beak: — Pentamerus (Gypidia, in part, Dalman). 

 Silurian to Carboniferous. 



A narrow area, and small foramen. Obscure internal septa : — Porambonites. 

 {Isorhynchus, King). Lower Silurian, Placed by McCoy, on account of the 

 area, in the family of the Oi'thisidas. 



T&rebratuUdce : — Shell punctate. Entering valve with internal shelly loop. 



Shell with prominent internal septa. Brachial support large and more or 

 less complicated. Area ; deltidium of two pieces ; and triangular or round 

 foramen : — Sikngocephalus. Devonian. Distinguished from Pentamerus by 

 its punctured shell, area, deltidium, (fee. Closely allied to the nest genus, 

 Terebratella. 



Shell with internal septum, and double loop. Area and foramen : the latter 

 truncating in part the beak of the receiving valve. Hinge-line nearly straight : 

 — Terebratella. (Rhynchora, Dalman ;, Magellanica, Chemnitz ; Belthysidia. 

 McCoy; Trigonosemus, Kceniz ; Terebrirosfra, Fissurirostra, &' Ovh. ; Magas, 

 Sow ; Lyra ; Bouchardia, Morrisia, Kraussia, Kingena, Davidson ; Ismenia, 

 Megerlia, King). Triassie (?) upwards. 



Internal septum absent, or more or less rudimentary. No area. Hinge-line 

 arched. Beak truncated by circular foramen: — Terehratula,mc^-a^\ng Tere- 

 bratulina, d'Orb. ; Epithyris, Phil. ; JEudesia and Waldheimia, King. The 

 latter are aberrant types approaching Terebratella). Devonian, upwards. 



Appendix : — Seminula, McCoy (Epithyris, King). Pala30zoic Terehratula, 

 without any deltidium. 



Shell thick, with one or more sub-marginal septa in entering valve. Hinge- 

 line straight. Foramen large : — Argiope {Megathyris, d'Orb.) Cretaceous, 

 upwards. 



Thecidem : — Shell, thick, punctate ; with complicated internal processes and gran- 

 ulose margins. No foramen. (Attached by beak) : — Thecidea. Triassie — 

 upwards. 

 D'Orbigny and McCoy place the Family of the Thecideae amongst the Rudistes ; 

 whilst many palaeontologists suppress it as a family, and arrange Thecidea with 

 the Terebratulidas. Pictet, in the last volume of his Paleontologie (2nd ed.) issued 

 a few months ago, separates it again from the latter, to place it between Terebra- 

 tulidas and Spiriferida. Its true position, however — ^passing from lower to higher 



