MA.NUAL OP THE MOLLUSCA. 



Rensselceria, if not synonymous witli, is closely related to, 

 Meganteris. 



Fossil, 11 species. Silurian to Devonian. Em-ope, North 

 America. 



Centkonella, Billings, 1859. 



Etymology, diminutive of l^entron, a spur. 



Type, Eliynclionella glans-fagea, Hall. 



Shell having the general form of Tcrehratula. Dorsal valve 

 with a loop consisting of two riband-like lamellse, which were 

 united at an acute angle at the point of greatest extension, 

 whence they recurve in a thin vertical plate which is not 

 attached at either margin, approaching in some respects to 

 Waldheimia. 



Distribution, 4 species. Devonian. North America. 



Leptoccelia, Hall, 1859. (Coelospira, Hall). 



Appears to differ from Centronella only in consisting of 

 Bpecies which have the surface ribbed instead of smooth. 



Bistrihution, 9 species. Mid. Silurian — Devonian. Europe, 

 North America. No true Terebratulse have been found in 

 beds older than the Devonian. 



Family II. — Spirieeiiid^.* 

 SYBUfGOTHYBis, Winchell, 1863. 



Fig. 19. Section through the beak of the ventral valve of S. typa (Winchell). 

 I, dental plates or lamellas ; t, tube incomplete ; r, mesial ridge. 



Examples, S. typa, Winchell, Eig. 19; Spirifera distans, Sow. 



Shell like that of Spirifera, with an elongated hinge-line. 

 Ventral valve with a broad mesial sinus, a very broad area, and 

 a narrow triangular fissure closed towards the apex by an 

 external convex pseudo-deltidium ; beneath which, and diverg- 

 ing from it, is another transverse plate connecting the vertical 

 dental lamellae, which are incurved so as to nearly join their 

 inferior edges, thus forming a fissured tube, which projects 

 beyond the limits of the plate from which it originates into the 

 interior of the shell. A low median ridge extends from the 



See p. 271. 

 54 



