Report of the State Geologist. 609 



penetrate obliquely the substance of the shell along the cardinal 

 line, converging toward the apex of the valve till they reach the 

 surface, where they turn at an abrupt angle upward and outward 

 and are thence continued as hollow spines. Cardinal teeth 

 strong. A low median ridge, slightly thickened at its posterior 

 extremity, where it is sometimes coalesced with the deltidium, 

 extends forward, dividing the muscular area. The muscular 

 scars are usually faint, and consist of flabelliform diductors 

 which partially inclose elongate median adductors. 



In the brachial valve the cardinal area is very narrow, and 

 without spines. The deltidium is partially developed, resting 

 against the cardinal apophysis. This process is very similar in 

 character to that in Plectambonites, consisting of a median 

 portion, coalescing at its base with the elongate crural plates. 

 On its posterior face it is divided by a narrow median furrow 

 and two broader lateral grooves, giving it a quadrilobate appear- 

 ance. The crural plates are slightly divergent from the hinge- 

 line, bounding narrow, elongate sockets. A median ridge 

 separates the quadruplicate muscular impression and from 

 between the anterior and posterior members of this impression 

 originate two linear brachial ridges, which extend outward, 

 recurving toward the median line at about the middle of the 

 valve, making a reniform curve. Interior surface of both valves 

 strongly papillose in the pallia! region. 



External surface usually covered with radiating stria?, rarely 

 smooth or concentrically rugose. Shell-substance fibrous- 

 punctate. 



Type, Chonetes striatella, Dalman (sp.). 



Distribution. Upper Silurian — Carboniferous. 



Subgenus Anoplia, Hall. 1892. 

 (Plate 20, Figs. 15-19.) 



Shells small with smooth or squamous exterior. No external 

 evidence of cardinal spines, though the spine-tubes exist in one 

 or more pairs in the substance of the pedicle-valve. On the inte- 

 rior a median septum in the pedicle-valve. Cardinal process and 

 muscular impressions of the brachial valve as in Chonetes. 



(Type, A. nucleata, Hall (sp.). Oriskany sandstone = Lower 

 Devonian.) 



1891. 77 161 



