318 Mr. J. Morris on the genus Siphonotreta. 
merely provisional, having had but limited opportunities of in- 
specting specimens, and having seen but three of the four ge- 
nera above described, and not any showing interior structure. 
In the collection of Sir R. I. Murchison are specimens of Sipho- 
notreta and Obolus which I have been kindly allowed to examine : 
for the loan of Orbiculoidea, D’Orb., Tam indebted to Mr. J. Gray 
of Dudley : with regard to Acrotreta, I have not seen the Russian 
specimens which are included in that genus, but the excellent 
figures given by Kutorga lead me to infer that they most pro- 
bably belong to that section of the Spirifere constituting the 
genus Cyrtia of Dalman, for the mesial furrow traversing the 
depressed triangular area in two of the species figured (4. sub- 
conica and A. recurva) indicates a more complex arrangement on 
the hinge-lne than is found in the hingeless Brachiopods. 
As to the peculiar structure of the shell of Siphonotreta, which 
is a character of some value and at once distinguishes it from the 
other genera, it is not a little remarkable that neither M. de Ver- 
neuil nor Dr. Kutorga has figured it or alluded to it with suffi- 
cient importance. De Verneuil describes the shell as subcorneous, 
a surface chagrinée. Kutorga states the calcareous part proceed- 
ing from the apex to consist of a number of very flat rings or of 
many oblique cones truncated at the bases, whereas Sir R. Mur- 
chison’s specimens of Siphonotreta exhibit, certainly a shell both 
calcareous and corneous, but with a distinctly perforated struc- 
ture, as if composed of a series of oblique tubular layers, the 
perforated texture being larger than that found in the majority 
of Terebratule, and resembling that presented in Ter. Capewelli 
(Davidson), Ter. hamifera (Barr.), in the genus Trematis (Sharpe), 
and in some species of Thecidea ; besides which the surface is 
ornamented, in all the described species, with numerous tubular 
spines, generally arranged in a very regular order, and leaving, 
when broken off, slightly projecting hollow tubercles im their 
place*. Neither of these characters are found in Orbiculoidea, 
D’Orb. (Schizotreta, Kut.), and Obolus, Kichw. (Aulonotreta, 
Kut.) ; their shells, although more solid and calcareous than the 
recent allies, are probably formed somewhat as in Orbicula and 
Lingula, and which are described by Dr. Carpenter as being 
“ almost entirely composed of laminz of horny matter, which are 
perforated by ninute tubuli, closely resembling those of ivory 
in size and arrangement, and passing obliquely through the 
laminee.” 
The genus Schizotreta, Kut., is synonymous with Orbiculoidea, 
D’Orb., and presents some, but probably only minor, characters 
* The genera Chonetes and Productus are also furnished with tubular 
spines ; in the former they are arranged along the cardinal margin of the 
dorsal valve, and in the latter are irregularly scattered over the surface. 
