412 
THE GEOLOGIST. 
development of the arm itself, or of its connecting membrane ; and it is 
therefore to bo presumed that something similar did occur among the 
Bpecies with spiral arms. 
The question relating to the classification of the species with spiral 
supports is still a matter which will demand some further consideration ; 
but it seems to me that the genera and sub-genera may be, for the 
present at least, arranged into the following separate groups : — 
' Genus Spikifera, Sow., type Sj}. striata, Martin. 
? Sub-genus, Cyrtia, Dalman, C. expoirrecta, Walil. 
\< J, Spirifcrina, WOvh. , Sp. rostrata, ^ic\Ao^h. 
„ Suessia, E. Desloug-., (S. costafa, Desl. 
„ Cijrtina, Dav., C. septosa, Pliillips. 
[Genus Athyris, WCoj=Spirigera d'Orb. A, conccntrica, V. 
Buch. 
Sub -genus, Mcrista, Suess, M. Ilerculea, Barraudc. 
„ „ Retzia King, It. Adricni, De Vern. 
„ „ Trcmatospira, Hall, T. perforata, ILilI. 
., „ Nuclcospira, Hall, N. ventric.osa, Hull. 
„ „ UnciteSflDet, U. Qryph^is,Y)&i. 
g C Genus Atrypa, Dalman=)Sptri/eJ7'/ja, d'Orb., A. reticularis, 
f Genus Koninckina, Suess, K. Leonliardi Weissmau. 
4 j „ Ajjoi'lotueca, Sandberger, A. lamellosa, Sandb. 
( „ Davidsonia, Bouchard, D. Verneuilii, Boucli. 
Nos. 1, 2, and 3 have been grouped by myself and others into a single 
family, Sjnriferidce, but Mr. Woodward has excluded the third, which 
he has added to the Rhjnchonellidce. 
A family, Davidsonidw, was proposed some years ago by Professor 
King, in which some authors would comprise Anoplotheca and Koninckina. 
The characters which distinguish Sjnrifera, Athyris, and Atrypa have 
been so clearly defined, that there can exist'no possible doubt as to the 
propriety of maintaining all three as Vv'cll-established genera. 
The shells which compose Sowerby's remarkable and widely-spread 
genus, SriEiPEEA, diff'cr exceedingly in their external shape, some being 
transverse, while others are elongated, rounded, smooth, angular, 
variously ribbed or striated, with or without a mesial elevation in the 
smaller valve and a sinus in the larger one, all possessing, however, a 
straight hinge-line, but very variable in length ; the larger valve is pro- 
vided with a flat or concave, triangular or sub-parallel area, which varies 
in height, width, and extent, according to the species ; the area is 
