156 



TWENTIETH REPORT ON THE STATE CABINET. 



Meristella arcuata. 



Showing the first volution of the spires, with loop, etc. 



Dorsal view of the central parts of the spiral lamellse. 



In these figures, a indicates the origin of the crura ; h the recurvation 

 of the lamellge, which may be as represented, or with a simple retral bending 

 without recurving upon itself in some species; c is the continuation of the 

 lamellae, which at d give off the processes forming the loop, and these 

 become united at e and continue simple to /, where they bifurcate and con- 

 tinue in the direction g^ returning again to the centre, and reuniting with 

 the loop at A, or near the junction of the two parts before mentioned. 



These characters of the spires and loop have been observed in the M. 

 IcEvis, M. arcuata and M. princeps of the Lower Helderberg group, and in 

 the M. harrisi of the Hamilton group ; while the M. nasuta presents a 

 slight modification in the extension of the parts of the loop, which ally it 

 more nearly with Athyris. 



When compared with the spires of Athyris as shown in the figures 

 already given under that genus, as well as the illustrations of Davidson 

 and other authors, the differences are obvious. The parts of the loop in this 

 genus, instead of curving forward and there uniting and turning backward 

 and bifurcating, to form the accessary lamellae, are continued from their 

 origin obliquely backwards into the cavity of the ventral valve, and, then 

 recurving upon themselves, are reunited laterally; while in Athyris they 

 are intercalated between the first and second turns of the spire, and coalesce 

 with the lamellae of the latter. 



At the same time, an examination of the Niagara species referred by me 

 to the Grenus Meristella presents a different condition of the interior, 

 and shows the lamellae of the spires united by a simple loop only. 



This feature is illustrated in the accompanying diagram, which represents 

 a single turn of the spiral lamellae, from their origin at <2, to i where the 

 bands are cut off. The thickened bases of the crura are represented at a ; 

 h is the point of recurvation, c the continuance of the lamellae in the dorsal 

 cavity, and d the expansion on the inner side into long processes which 

 unite at e, forming the loop. 



