PROSOBRANCHIATA. 



7/3 



Neritidce. In this genus the shell is discoidal, of few whorls, longi- 

 tudinally grooved at the back, and ornamented with lines of growth. 



Fig. 662. — Caloccntrus {Cirrus) Goldfussi. Devonian. 



The upper side of the shell (drawn as the lower side in fig. 663, b) 

 is convex, and deeply umbilicated by the sinking of the spire below 



Fig. 663. — Maclurca crenulata, Ordovician (Quebec Group), Canada, a, Upper side, with 

 the spire; b, Side view of the shell; c. Base. Properly speaking, figure b should have been 

 drawn with the flattened base undermost and the hollow spire above. (After Billings.) 



the general surface ; while the base is flattened and exposes the inner 

 whorls. The mouth of the shell is closed by a shelly operculum, 



Fig. 664. — Pleuronotus {Euomghalus) De-Ceiui (Billings), a, Front view ; b, View of the 

 umbilicus. Devonian, North America. 



which is " sinistrally sub-spiral, solid, with two internal projections, 

 one of them beneath the nucleus, very thick and rugose " (Wood- 

 ward). The genus is essentially characteristic of the summit of the 



