160 GEOLOGY AND MINERALOGY. 



The relation of these families might be better shown by the following ai-range- 

 ment — the thick-shelled crania being evidently somewhat related to the Theeidese 



as well as to the hingeless types 

 which precede it in the grouping. 

 Calceola stands apart, although 

 with much that connects it, as an 

 aberrant form, to the Orthisidee. 

 The ThecideEB — in many respects, 

 aberrant types of the Terebratulidse 

 — form a transition-group to the 

 Rudistes. See, further, the re- 

 marks at the close of Table II. 



Lingulidae. 

 Discinidee. 



Graniadse. 

 Calceolidse. 

 Orthisidse, 

 Spiriferidse. 

 Rhynconellidas. 

 Terebratulidas. 



Thecidese. 



Table II. — Genera of the Brachiopoda. 



Lingulidm : — Shell corneous, or sub-corneous ; nearly equivalve ; hingeless ; no 

 true perforation. 



Oblong, horny : — Lingula (including Glossina, Phillips). Lower Silurian, 



upwards. 

 Orbicular or sub-orbicular*; calcareo-corneous : — ObolusiUngula, Pander; 

 Aulonotreta, Kutorga). Silurian. 

 Discinidce : — Shell corneous, or sub-corneous ; hingeless; longitudinal slit or fora- 

 men in one valve. 



Shell impunctate: — Discina Wrbicula, including also Orbiculoidea, 



d'Orb, the Schisotreta of Kutorga). Lower Silurian, upwards. 

 Shell with surface punctures : — Trematis ( Orbicella. d'Orbigny). Silurian. 

 Shell coarsely punctured aud spiny : — Siphonotreta. Silurian. 



Appendix: — Acrotreta, Kutorga. Allied, according to Morris, to 

 Gyrtia, in Family of the Spiriferidce. 

 Craniadce : — Shell thick ; hingeless ; orbicular ; punctate ; without foramen. 

 Crania (including Pseudocrania, McCoy). Silurian, upwards. 

 Appendix: — Spondylobolus, McCoy. 

 Calceolidce : — Shell thick, fibrous. Receiving valve large and conical. Hinge-line 

 straight, with row of obscure teeth. 

 Calceola. Devonian. 

 Orthisidce: — No internal shelly process ; shell punctate; hinge-line sti'aight. 



§ 1. Shell bi-convex, or plano-convex; with area and foramenf. Greatest 

 width generally below the hinge-line. 



Triangular foramen ; no deltidium : — Orthis. Silurian to Garb, (or Permian ?) 

 This genus includes many so-called Spirifers and Terebratulidse ; and also, in 

 part, Orthambonites and Gonambonites of Pander, and Dicoelosia, Platystro- 

 phia, and Schizophoria of King. 



Round foramen, unless closed by age ; deltidium (or pseudo-deltidium) :— 

 Orthisina (Pronites, Hemipronites, Pander ; Streptorhynchus, King). Silurian 

 to Permian. 



* McCoy and other paleeontologists place some species of Obolus under Lingula— thus 

 making orbicular as well as oblong lingute. 

 t Sometimes closed or rendered obscure by age. 



