290 



T. Davidson — On the Genera Merisfa and Dayia. 



internal and differential characters were then likewise unknown. 

 In our British Devonian rocks the genus Merista is represented by 

 the Merista plebeia, or =z scalprum ; but although Mr. Glass has been 

 able in several specimens to develope its spirals, he entirely failed to 

 expose its loop, on account of the difficult nature of the limestone or 

 opaque spar which fills most of the specimens. 



Considering it to be very desirable that the interior character of 

 this important genus should if possible be discovered, I wrote to my 

 old and valued friends M. Barrande, of Prague, and to Prof. E. 

 Suess, of Vienna, and requested them to kindly send me some 

 specimens of Merista Herctdea, that tlie Eev. Norman Glass might 

 endeavour to work out the loop and the attachments to the hinge- 

 plate, which had not been hitherto discovered; and thanks to his 

 great skill, experience and patience, we are now acquainted with 

 the whole characters belonging to the genus under description. 



The principal stems forming the spirals are attached to the hinge- 

 plate a ; from thence they proceed for a short distance into the interior 



of the shell with a very 

 gentle inclination forward, 

 and at b they are abruptly 

 bent backwards at an acute 

 angle towards the bottom 

 of the lateral portions of 

 the beak. From thence 

 they form a broad rounded 

 curve facing the bottom of 

 the dorsal valve c, and after 

 converging to about half 

 their length, again divide 

 towards the front, and thus 

 form the first spiral coil. 

 Again, at about half their 

 length, at d, the principal 

 lamellc© widen and give off 

 another lamella. These 

 lamellee converge from 

 both sides towards the middle of the interior of the shell between 

 the spiral coils, and after the two extremities have come into contact, 

 the lamella thus formed proceeds in a straight direction for a short 

 distance to near the hinge-plate, and then bifurcates and curves 

 round on each side, forming two slender rings e. The anterior 

 border of these rings being attached a little below the place where 

 the converging lamellee of the loop become united. The outer edges 

 of the rings slope gently towards the bottom of the dorsal valve, 

 and are rather less in width than the primary branches to which they 

 are attached. The spiral cones are composed of ten or twelve 

 convolutions, the number, however, varying in different specimens 

 and at different stages of growth. The extremities of the spirals 

 are directed towards the middle of the lateral portions of the shell. 

 In the ventral valve, under the beak, are two roof-shaped plates 

 fixed by their lateral margins to the medio-longitudinal region of the 



Interior of the dorsal valve of Ilerista Herculea. 

 Developed by the Eev. Norman Glass. 



