EXPLANATION OF PLATE L H. 



1 PSEUDOCRANIA DIVARICATA (M"^Coy). From tlie Upper Bala rock of Bryn Melyn quarry, Bala. 

 Natural size of exterior. 



1 a Portion of superficial markings magnified. • 



2 Cast of interior, shewing the two pairs of muscular impressions projecting. 



2 a Portion of ditto, shewing the impressions of the pallial vessels magnified. 



.S SIPHONOTRETA MICULA (M'Coy). Several specimens, natural size, on portion of shale, from three miles 

 North of Builth. 



3 a Perforated valve magnified four diameters. 



4 SPONDYLOBOLUS CRANIOLARIS (M'Coy). Group of three casts of inner surface of large valve, natural 



size, from the black shale of Builth Bridge. 



4 a Wax cast from one of ditto, shewing the appearance of the interior of the shell magnified. 



5 Exterior of small specimen of small valve. 



5 a Interior of ditto, shewing the thickening of the shell beneath the apex. 



5 b Portion of external surface very highly magnified. 



6 HEMITHYRIS ANGUSTIFRONS (\PCoy). Natural size, from the sandstone of Dalquorhan, shewing 



the entering valve and the foramen beneath the beak. 



6 a Profile. 



7 Internal cast of receiving valve. 



8 Internal cast of entering valve. 



9 HEMITHYRIS SUBUNDATA (M^Coy). Natural size, from the limestone of Mathyrafal, shewing the 



smooth shell, partially removed, exposing the dichotomous impressions of the pallial vessels. 



9 a Profile of rostral portion. 



9 b End view of internal cast. 



9 c Front view, shewing the elevation of the margin. 



10 HEMITHYRIS HEMISPH^RICA, var. SCOTICA (M-^Coy). Natural size, entering valve, from the sandstone 

 of Mulock, Dalquorhan. 



10 a Receiving valve. 



10 b View of front margin. 



11 ORTHIS HIRNANTENSIS (MToy). Natural size of rather small specimen, from the Bala hniestone 



of Aber Hirnant. 



11a Portion of surface of ditto, magnified to shew the fasciculated striation. 



116 Longitudinal section. 



lie Portion of internal cast of receiving valve, shewing the hinge teeth. 



12 ORTHIS RETRORSISTRIA (M=Coy). Natural size, from the flags of Cerrig y Druidion. 

 12(1 Profile of both valves. 



12 b Half the hinge line and part of side magnified to shew the backward arching of many of the stria; to 



terminate along the hinge line. 



1.3 Cast of interior of entering valve, shelving the rostral and cardinal teeth and the quadrate muscular 



impressions. 



14 Internal cast of receiving valve, shewing the very long divaricating muscular impressions. 



15 ORTHIS SAGITTIFERA (M-'Coy). Natural size of rather small specimen of receiving valve, from the 



schists of Aber Hirnant. 



15 a Portion of surface magnified. 



16 Rostral portion of entering valve, shewing the mesial sulcus, cardinal area, and foramen. 



17 Internal cast of entering valve, shewing the broad-arrow-shaped mark produced by the sulci of the cardi- 

 nal and mesial plates. 



17 a Internal cast of rostral portion of receiving valve, shewing the muscular impressions. 



18 Longitudinal section of both valves. 



19 Transverse section. 



20 ORTHIS TURGIDA (M'^Coy). Natural size, from the limestone of Llandeilo. 



20 a Portion of surface magnified. 



21 Internal cast of receiving valve, from Goldengrove. 



