308 



PALAEONTOLOGY OF NEW- YORK. 



Tbese features are illustrated in the accompanying diagrams ; fig. 1 being an oblique lateral 

 Ticw of the central portion of the spires, and fig. 2 a view of the same parts from the dorsal 

 aide.* 



MiRISTELT.A ABCUATA. MEBtSTELtA ARCtJATA. 



Showing the first volatlon of the spires, with loop, etc Dorsal view of the central parts of the spiral lamelln. 



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

 ■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 continue in the direction g, returning again to the centre, and reuniting with the loop at 

 h, or near the junction of the two parts before mentioned. 



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

 M.princeps of the Lower Helderberg group, and in the M. barrisi 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 Athyeis. 



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 Atiivkis, they are intercalated between the first and second 

 turns of the spire, and coalesce with the l.amella; of the latter. 



the generic name of CnARiONELLA; professing to "have ascertained the generic characters of the 

 so-called Alrypa or Athyris scitula," a species possessing the characters of Mkihstella, and none, 

 80 far as known, which do not belong to tliat genus. lie has subsequently united under the same name 

 several other species of the latter genus. It is scarcely necessary to add that the characters of hinge, 

 ss described and illustrated in the dorsal valve of Charionella by Mr. Billings, do not exist in any 

 species of this family (Spirifcrida:) of Brachiopoda. 



• The words upwards and downwards, used in the descriptions of the spires in Atutris and 

 Mbristella, have reference to the position of the figures only, which are represented with the dorsal 

 tide below ; since the illustrations could be more satisfactorily given in this position. 



