106 



SUPPLEMENT TO THE 



Meristella arcuata. 

 Hall. 

 Dorsal view of the 

 central parts of the 

 spiral lamellae (after 

 Hall). 



In external shape Meristella is said to be oval, ovoid, or suborbicular, elongate or rarely 

 transverse; valves unequally convex, with or without a median fold or sinus; beak much 

 incurved, sometimes with a small circular foramen ; surface smooth. In 

 the interior of the dorsal valve the primary stems of the spiral coils are 

 attaclied to a strong hinge-plate at a. After extending slightly down- 

 wards under the beak of the ventral valve, they suddenly bend back- 

 wards at and towards the lateral portions of the shell. From thence 

 (as in Merista and JFhitfieldia) they form a broad rounded curve 

 facing the bottom of the dorsal valve (<?), and after converging to about 

 half their length again diverge towards the front, and thus form the 

 first spiral coil. At about half their length (at d) the principal 

 lamellae widen and they each give off another lamella. These lamellae 

 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 cuter 

 sides of the rings slope gently downwards towards the primary branches to which they 

 are attached. 



Erom this description it will be seen how completely the loop and connections in 

 Meristella agree with those of Merista. Prof. Hall was not, however, acquainted with 

 the loop or its attachments in Merista, and consequently could not point out their 

 identity. 



Meristella differs, however, from Merista by the absence of a shoe-lifter process in 

 the ventral valve. It shows, as observed by Prof. Hall, " a triangular fissure below 



the beak, which joins a semicircular perforation at the apex and there is 



sometimes a thickening of the shell at the base of the rostral cavity, which limits 

 the muscular impressions ; but there is neither septum nor rudiment of one, as in 

 Meristar 



With his usual liberality and desire to afford help in our researches, Mr. Whitfield 

 forwarded to the Rev. N. Glass a specimen of Meristella arcuata, in which the spirals 

 and their connections were silicified, and Mr. Glass was able to dissolve the matrix 

 surrounding the lamellae and to expose the rings of the loop in the most clear and 

 beautiful manner. In the specimen developed by Mr. Glass the rings are not as 

 large as they are figured by Mr. Whitfield, that is to say, they do not appear to extend 

 beyond the outer margin of the primary stems of the spirals, but are rather within 

 them. 



I have likewise, thanks to Mr. Whitfield's liberality, been able to examine a developed 

 specimen of Prof. Hall's Meristella Barrisi from the Marcellus shale and Hamilton group 

 of Southern Indiana, and which showed the rings of the loops in great perfection. 



