98 OF CONCHOLOGY. 



Platidia. — Loop simple, not reflected ; attached to a me- 

 sial septum. No cardinal process. Foramen 

 encroaching on both valves. 

 MEGATHYRIN^. — Mouth central, surrounded hj a mem- 

 branous disk, bounded by non-spiral sub- 

 marginal brachia. 

 Megathyris.- — Hinge-line straight ; loop not reflected, 

 submarginal, attached. Mantle adherent to the 

 shell. 

 Megathyris. — Foramen and area large. Shell sculptured. 

 Loop attached to three or more submarginal 

 radiating septa. 

 Cistella, — Shell smooth or sculptured. Loop attached to 

 a single submarginal mesial septum, Avhich in- 

 terrupts the brachia. 



* ♦ * ♦ 



? THECIDIIN^.— Shell attached by the substance of the 

 neural valve, and with the foramen closed 

 when adult. Young with a minute apical, 

 foramen. Loop entirely absent. Mantle 

 supporting the brachia (which are non-spiral 

 and irregularly lobed), and itself supported 

 by a self-secreted irregular calcareous net- 

 work. 

 Thecidium. — Shell thick, oval, rounded. Area large and 

 flat. Crura united in a bridge over the visceral 

 cavity. Brachia usually separated by a mesial 

 ridare and its branches. Calcareous network 

 variable. 



Note. — Eicliwaldia, Bill, which is doubtfully referred by Bronn to the 

 Terehratulidce, is founded on a shell [E. subtrigonal/s, Rep. Prog. Can. 

 Sur. p. 192, 1858) of which the internal structure is unknown, but which 

 resembles externally, in the position of the foramen and general appear- 

 ance, Magasella Camingii, Davidson, pp., P. Z. 8. 185'2, pi. xiv, fig. 10 — 

 16. The genus was published in the Rep. on the progress of the Canadian 

 Geol. Survey, 1858, p. 192, fig. 24, a to e, and the type is from the Coal 

 Measures of Canada. 



Leptocoelia, Hall, is closely related to Atrypa. 



Hyrmiphoria, Suess, is unknown to me. 



Tropidoleptus has been referred doubtfully, by Prof. Hall, to the Tcre- 

 bratididce. The type is Strophomena carlnata, Conr. I do not feel suf- 

 ficiently confident of its true position to admit it at present. The apo- 

 physes appear to resemble a Magas, without the upper reflected portion. 



F?<i<Zi»a, Hall, is somewhat undecided in its relations; V. pustuloiu, 

 Hall, is the type.— T7t/e Pal. N. Y., vol. iv. 



