BRACHIOPODA. 



66 1 



area, in the centre of which is a deltoid foramen often partially closed by 

 a pseudo-deltidium ; while the dorsal valve is furnished with a prominent 

 median septum, which is continuous with the cardinal process superiorly, 

 and may be bifurcated at its anterior extremity. The species of Skeni- 

 dium are Ordovician and Silurian, and it seems probable that the genus 

 is identical with the Devonian Mystrophora of Kayser. 



In the Silurian and Devonian genus Tropidoleptus (fig. 515, A — c), the 



Fig. 515. — a, Dorsal view of the shell of Tropidoleptus carinatus, Devonian (Hamilton group) 

 of North America, of the natural size ; b, Interior of the dorsal valve of the same ; c, Interior 

 of the ventral valve of the same ; d, Dorsal aspect of Skenidium znsigne, from the Silurian 

 (Lower Helderberg) of North America ; e, Cardinal aspect of the same enlarged, showing the 

 high area of the ventral valve, and the triangular foramen partially closed by a pseudo-deltidium ; 

 f, Base of the same ; g, Interior of the dorsal valve of the same, showing the median septum. 

 (After Hall.) 



shell is concavo-convex, radiately ribbed, with a straight hinge-line and 

 a double area, that of the ventral valve perforated by a large foramen. 

 The ventral valve carries two strong, crenulated and diverging teeth, 

 and the dorsal valve has two correspondingly crenulated dental sockets, 

 a well-marked median septum, and a prominent cardinal process which 

 almost entirely fills the foramen in the ventral 

 valve. The species of Tropidoleptus are Silurian 

 and Devonian. 



In the genus Orthisina (= Klilambonites and 

 Hemipronites) the shell resembles that of Ort/tis, 

 but the ventral valve is pyramidal, and has* a 

 very high area, directed more or less at right 

 angles to the median plane of the valves, while 

 the dorsal valve is furnished with a smaller area. 

 In both valves there is a triangular fissure in the 

 hinge-area, covered by a pseudo-deltidium, that of 

 the ventral valve being often perforated by a peduncular foramen. 

 The typical forms of Orthisina are found in the Ordovician deposits. 



In the genus Strophomena (figs. 513, 514, and 517) the shell is 

 depressed, generally semicircular, the hinge-line as long as the width 

 of the shell, or longer. The surface may be smooth, but is most 

 commonly striated or ribbed. There is a double hinge-area, which 

 is largest in the ventral valve. Each hinge-area has a median notch, 



