104 



SUPPLEMENT TO THE 



Merista Serculea, Barrande. 



Interior of the dorsal valve, exposed by the 



Eev. Norman Glass. 



the shell between the sph-al coils, and after the two extremities have come into contact 



the lamella thus formed proceeds in a straight direc- 

 tion 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 lamellae of the loop become 

 united. The outer edges of the rings slope gently 

 towards the bottom of the dorsal valve. The rings 

 are rather less in width than the width of the primary 

 branches to which they are attached. The spiral 

 cones are composed of ten or twelve convolutions, 

 the number, however, varying in diflFerent 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 valve, and with their narrow end 

 fitting under the extremity of the beak. Prof. King compared these plates to a shoe- 

 lifter. 



With very small differences the loop of Meristella, Hall, is similar to that we have 

 described in Merista ; and, were it not that Meristella has no shoe-lifter process, it would 

 be impossible to distinguish the two genera. Again, Whitjieldia tumida is distinguishable 

 from both Merista and Meristella by the absence of those peculiar ring-shaped processes 

 attached to the loop, and has instead only a short bifurcating process where in both 

 the last-named genera the rings are formed. These three genera seem, indeed, closely 

 allied, although each contains peculiarities by which it may be distinguished from the 

 others. I am also indebted to Dr. C. Rominger for the sight of a specimen of Merista 

 Herculea he has endeavoured to develop, but in which, although he succeeded in exposing 

 the spirals, he failed in obtaining the attachments of the spirals or loop. 



Mr. R. P. Whitfield writes me that in American Silurian rocks the genus Merista 

 is hitherto known to be represented by one species only, namely, the Merista {Camarium) 

 typus, Hall, of which the M. elongata is a variety. It is consequently as rare in 

 that part of the world as it is in Great Britain. In Bohemia, on the contrary, the 

 species of the genus are numerous, and are figured in Barrande's magnificent work on 

 the Silurian fossils of that kingdom. 



The genus Merista appears to be largely represented in the Devonian and Silurian 

 rocks of different countries, although rare in Great Britain. Since the discovery made 

 by Mr. Glass of the complete interior of Merista Herculea, its rings, connections, «S:c., 

 Herr Zugmayer has devoted considerable time and patience to developing the interior 

 of several specimens of Quenstedt's Merista cassidea-prunulum from the Devonian rocks 



