14 BULLETIN OF THE LABORATORIES 
1. Genus Spirifer. Elongate transversely, hinge-line longest lateral 
dimension, radiatingly ribbed or plicate, somewhat tri-lobed. 
2. Genus Syringothyris. Like Spirifer, with fold and sinus smooth, 
area of ventral valve high, area with a perforated deltidium lead- 
ing into a funnel-like projection into the interior. 
3. Genus Spiriferina. Small spirifers with few plicae and a promi- 
nent partition in the ventral valve. 
4. Genus MaHinia. Hinge-line shorter than greatest width, sur- 
face spiny and concentrically grooved. 
5. Genus Cyrtma. Like Spiriferina but with very high ventral 
valve, prominent deltidium closing foramen. 
6. Genus Athyris. Transversely oval, concentrically striate, area 
obsolete, beak perforate, spires as in Spirifer. 
7. Genus Rhyruhospira. Surface striate or plicate. 
V. Earn. Strophomenid^. 
Shell transversely oblong, with long hinge line and small areas, 
generally piano- or concavo-convex. 
1. Genus Orthis, Hinge shorter than width, radiately striate, bi- 
convex. 
2. Genus Hemipronites . Concavo-convex, hinge nearly as long as 
greatest width, area of ventral valve narrow, with deltidium, rad- 
iating striae. 
3. Genus Strophomena. Depressed, concavo-convex, area on both 
valves, radiating striae. 
V. Earn. Production. Concavo-convex, with straight hinge, gener- 
ally shorter than greatest width of shell, surface or hinge with 
long spines, surface porous. 
1. Genus Productus. Whole surface with spines. 
2. Genus Chonetes. Spines restricted to the hinge. 
Genus Chonetes. 
A. Of large or medium size, gently convex. 
a. 
Surface marked by over 100 ribs. 
b. 
Surface marked by 40-60 ribs. 
1. 
2. 
C. MULTICOSTA. 
C. ILLINOISENSIS. 
B. 
Very convex, of medium size. 
3. 
C. LOGANI. 
C. 
Of rather small size. 
4. 
C. TUMIDUS. 
