CLASS IV 



GASTROPODA 



521 



Tryhlidium Lindstrom (Fig. 845). Shell depressed, very thick, oval, with anterior 

 beak ; ornamented externally with concentric lamellae. Six pairs of muscle scars 



arranged in the form of a hoise-shoe. 



Ordovician and Silurian. 



Family 4. Cyrtolitidae UMch and 

 Scofield. 



Symmetrical, involute shells with two or 

 three volutions, barely in contact, sharply 

 angular dorsally ; aiierture not expanded, the 

 sinus V-shaped, never deep, sometimes want- 

 ing ; slit absent ; surface reticulate. Ordo- 

 vician and Silurian. 



Gyrtolites Conrad (Fig. 846, A, B). Shell 

 carinated on the back and often on the 

 sides, giving a sub-quadrate cross section ; no 

 slit band. 



Cyrtolitina Ulrich. Small thin shells 

 with a slit band. 



Microceras Hall. 



c 



Pig. 845. 



Tri/blUUum reticukttum Lindstrom. Silurian; 

 Gotland. .4, Internal, and li, external aspect 

 (after Lindstriim). 



D 



Fig. 846. 



A, B, Cyrtolites ornatas Conrad. Ordovician of Boonville, New York, and Cincinnati, Ohio. C-E, SinuUes 



rancellatus (Hall). Ordovician of Minnesota. 



Family 5. Sinuitidae, novum. (Protmvarthiidae Ulr. and Scof.). 



Symmetrical, involute shells with aperture not abruptly expanded ; outer lip and 

 lines of growth loith a broad or narrow dorsal sinus ; slit and band wanting. 



Sinuites Koken (Protowarthia U. and S.) (Fig. 846, C-E). Aperture large with 

 outer lip bilobate ; dorsum convex ; umbilicus closed. Ordovician to Devonian. 

 Bucanella Meek. Dorsum of shell trilobate ; umbilicus large. Silurian. 

 Owenella Ulr. and Scof. Cambrian. 



Family 6. Bucaniidae Ulrich and Scofield. 



Symmetrical, involute shells with rather numerous lohorls merely in contact or 

 embracing slightly, all visible in the umbilicus ; aperture often expanded abruptly ; dorsal 

 slit band distinct with the slit long and narrow; surface with transverse lamellae or 

 lines usually crossed by short ribs. Ordovician to Devonian. 



Bucania Hall (Fig. 847, A, B). Shell of three to five depressed volutions coiled 

 in a plane, generally with a wide umbilicus and with aperture never abruptly 

 expanded. Ordovician and Silurian. 



Salpingostoma Roemer (Fig. 847, C). Aperture abruptly expanded, trumpet-like; 

 outer half of last whorl with a long, narrow slit closed some distance behind the 

 apertural expansion. Ordovician and Silurian. 



Trematonotus HaU. Like Salpingostoma except that the slit band is replaced by 

 a row of perforations. Silurian and Devonian. 



