590 Forty-fifth Report on the State Museum. . 



plates small ; the latter appear to unite and form a low elevation 

 across the base of the delthyrium. Cardinal process absent or 

 rudimentary in all the specimens examined. Muscular markings 

 in both valves extremely obscure. 



Surface marked by distinct plications, with interstitial finer 

 radii, which are crossed by delicate concentric striae ; these are 

 usually accompanied by a low sinus and fold on the brachial and 

 pedicle-valves, respectively ; interior very finely papillose. Shell- 

 substance fibrous and apparently punctate. 



Type, Orthis Billingsi, Hartt. (Cambrian.) 



Clitambonites, Pander. 1830. 

 (Plate 8, Figs. 8, 9.) 



Synonyms ; Pronites, Gonambonites, Pander. 1830. 



Shells with a subsemicircular marginal outline*; convex or 

 subpyramidal in the typical group. Hinge-line straight and 

 forming the greatest diameter of the shell. Pedicle-valve 

 elevated, cardinal area high, vertical, or sometimes incurved, and 

 crossed by a broad delthyrium, which is normally Covered by a 

 convex, perforate deltidium. On the interior of the valve the 

 dental lamellae are very strongly developed, converging and 

 uniting in the median line before reaching the bottom of the 

 valve ; thus forming a spondylium, which, with the deltidium, 

 incloses a conical subrostral vault. This plate is supported by 

 a median septum extending for about one-half the length of the 

 valve. Muscular impressions obocure. In the brachial valve 

 the cardinal area is considerably developed, and the delthyrium 

 filled by a conspicuous callosity, against the inner side of which 

 the simple orthoid cardinal process abuts. The dental sockets 

 are large, the crural plates low and continuous with the edges of 

 the delthyrial callosity. A thickened transverse area is formed in 

 the umbonal region by the union of the inner portions of the 

 crural plates with the cardinal process, and thence a broad median 

 ridge is continued forward through the muscular area, which is 

 sharply defined and quadripartite. External surface covered 

 with radiating striflB. Shell-substance impunctate. 



Type, Clitamhoni1< 8 adsoendens, Pander. 



Distribution. Lower Silurian. 



142 



