STRATIGRAPHY OF THE SKYKOMISH BASIN 



58i 



by Professor M'Corq (quoted in Hall, op. cit.). In these specimens 

 the most convex shell (Fig. 11, A, B) is not sharply flexed at any 

 one point, while younger individuals, as in Fig. 11, C, show less 

 arching, but a more sudden geniculation. The specimens are partly 

 exfoliated and have a finely punctate surface. 



Fig. 11, A, B, and D are of pedicle valves, and Fig. 11, C of a 

 brachial valve. This last specimen is partly an impression of the 



Fig. 11. — Rafinesquina (Hall) deltoidea (Conrad). A, B, and D, pedicle valves; 

 C, brachial valve. 



outer surface of the shell, since the striations show as grooves; 

 but part of the true shell remains where the muscle scars are shown. 



Illaenus americanus (Billings) 



Billings (1859, quoted; 1865, quoted and figure copied): Paleozoic Fossils, 



I, 329; Fig. si6a-d. 

 Winchell and Ulrich: Minn. State. Surv., Ill (1897), Part II, 714; Figs, (from 



above) 20, 21, 22, 23. 



A C B 



Fig. 12. — Illaenus americanus (Billings). A, front view; B, top view; C, side 



A single specimen, the glabella and fixed cheeks, was found 

 associated with the Rafinesquina deltoidea. All the features on 

 these parts agree perfectly with Billings' figures. This specimen 

 probably belonged to an animal one inch long, while Billings' 



