Report of the Geologist. 605 



Triplecia, Hall. 1 

 (Plate L8, FlgB. l L5.) 

 S\ ium\ in : Diora '/■ - 1 . 1 v 7l\ 



shell trilobate, transverse, unequally biconvex. Hinge-line 

 straight and quite short Pedicle valve shallow, convex about 

 the beak, bul depressed anteriorly by a broad and deep median 

 sinus ; cardinal area low, erect and well defined; delthyrium 

 covered by a narrow, convex plate, with a circular foramen at 

 the apex. In the interior the teeth are well developed and sup- 

 ported by short dental lamella' Longitudinally dividing the 

 umbonal cavity near its apex. Muscular area small, comprising 

 two lateral sears, separated by a longer central adductor impres- 

 sion. The brachial-valve is very con vex. and bears a strong median 

 fold. The cardinal area is very narrow and the beak closely 

 incurved. In the interior is a long-, erect and bifurcate cardinal 

 process, the distal extremity of each branch bearing a single 

 deep groove. This process is supported on a subrostral callosity, 

 which also bears two short spiniform crural points at its base. 

 Shell-substance fibrous, impunctate ( \ ). Surface with obscure 

 concentric growth-lines, and fine radiating striae on the inner. 

 laminae ; in rare instances there are radiating lines on the exterior. 



Type. Tripl tems, Emmons, (sp.) 



Distribution. Lower — upper Silurian. 



Mimulus, Barrande. 187!*. 



(Plate 18, Figs. 16-19.) 



Contour trilobate as in Triplegia median fold on the 

 pedicle-valve in the typical species. Shells sometimes bilaterally 

 unsymmetrical. Cardinal area of the pedicle-valve smooth, with 

 but little if any external evidence of deltidium. Teeth small, 

 not supported by dental lamellae. Brachial valve with an erect, 

 deeply bifurcated cardinal proces 



Type, Mimulus perve? f sus, Barrande. 



Distribution. Upper Silurian. 



Streptis, Davidson. 1881. 



(Plate 18. Figs. 20-22.) 



Shells biconvex, bilaterally unsymmetrical ; divided medially 



by a sinus. Pedicle-valve with a well-defined cardinal area and 



oramen. Teeth small, supported by short dental plates. 



Brachial valve with an erect, probably bifurcated cardinal 



157 



