146 



Elsewhere we have noted the enlargement of the species of a genus in 

 passing upward from one fauna to another, as, for instance, Acrotreta. 

 A similar enlargement in size may be noted in Obolus. 



Length. 



Obolus torrentis, Coldbrook terrane ............ 7 to 8 mm. 



O. -- triparilis, Lower Etcheminian ........... 8 to 9 > 



O. - discus, Lower Etcheminian .............. 10 u 



O. - sequiputeis, Lower Etcheminian ......... 11 to 12 n 



O. -- lens, Upper Etcheminian ................ 11 to 14 u 



O. -- Bretonensis, Upper Etcheminian ........ 14 to 17 " 



Var. LONGUS, n. var. PI. VII., Figs. 4a and b. 



s ^ nfc appears quite distinct from Obolus lens, but has many 

 described. points in common. It has the form of an oval Lingulella, and the posi- 

 tion of the muscle sears of the ventral valve is like that of Lingulella 

 ovalis ; nevertheless the peculiar sharp, deep and long mesian furrow on 

 back of the dorsal valve, and the ornamentation of the surface of the 

 valves is so like that of Obulus lens, that we have concluded to place it 

 as a variety of that species. 



The general form is lance-oval and the width two- thirds of the length. 

 Except in having shorter lateral vascular ridges, and in the difference of 

 proportion due to difference of form, this species does not vary much from 

 the type. It has the same mesian ridge in the ventral and a similar de- 

 pressed longitudinal band on the dorsal. It has the shallow grooves 

 along the mesian ridge of the ventral valve that is apparent in some 

 examples of Leptobolus collicia. Its thin shell, weak cardinal area, flat- 

 tened mesian band on the dorsal, and straightened lateral margins are 

 Leptobolian characters ; but it has distinct and rather coarse concentric 

 ridges on its surface, while in Leptobolus this sculpture is fine and 

 close. Michwitz appears to have met with similar forms that he had 

 difficulty in placing as varieties of Obolus (Schmidtia) obtusus. They are 

 of the Dictyonema Zone* 



Sculpture. This, consisting of close set concentric ridges, does not 

 differ from that of Obolus lens, except in being finer and closer. 



Size. Length of ventral, 16 mm.; width, 10 mm. ; depth, about 1^ mm. 

 The dorsal is about 1 mm. shorter than the ventral. 



Horizon and locality. Occurs infrequently in assise E. 3 e at Dugald 

 brook, Escasonie, N.S. 



The following are dimensions of valves of this form : 



*Uber die Brachiopodengattung Obolus, Eichwald, Mem. Acad. Imp. des Sci. St. 

 Petersb. Ser. viii., vol. iv., No. 2, Taf. ii., figs. 23-26 and 29-35. 



