188 PERMIAN FOSSILS. 
(vide Pl. XV, fig. 23) the adductor muscular impressions (a, 4), pedal or visceral ditto 
(d, e), and entire pallial line (0), are often instructively displayed. 
The cartilage of one of my specimens of Schizodus awxiniformis is rather long, 
slightly prominent, and situated immediately behind the point of the umbones, that is, 
on the dorsal slope of the longest side of the shell. ‘ 
Schizodus, Myophoria, and Trigonia are intimately related to each other. The 
- dental system appears to be but slightly modified in each genus. In Zirigonia, the 
teeth are massive, generally sulcated, and extending considerably into the cavity of the 
shell; the anterior ones are so far subject to the last condition, as to allow of the 
anterior adductor muscle being inserted before them. In d/yophoria, the teeth are 
somewhat reduced in size, and plain—at least, generally so; the anterior teeth also 
project considerably into the cavity of the shell, or rather become joined to the ridge, 
which posteriorly bounds the anterior adductor muscular impression; hence the deep 
groove, in front of the umbones, in casts of this genus: in Zigonia, the ridge may be 
said to be an integral portion of each anterior tooth. In Schizodus, the teeth are still 
more reduced, and quite plain; and there is a complete absence of the muscle- 
bounding ridge. The differences just named are accompanied with others, which 
are external. TZigonia is generally ornamented with ribs, tubercles, and varices, 
frequently combined: MMJyophoria is usually characterised with a variable number 
of longitudinal ribs: and Schizodus is, in general, simply marked with fine raised 
concentric lines. 
By means of its relation to Myophoria and Schizodus, the singular genus Zrigonia 
stands in a less isolated position in our conchological systems than at first sight it 
would appear ; it becomes, in fact, intimately connected with ordinary genera ; inasmuch 
as its massive sulcated teeth are seen to graduate almost insensibly into those charac- 
teristic of the normally dentigerous Dimyarians. 
All the known species of Schizodus belong to the Carboniferous and Permian 
formations. 
1 Dr. Geinitz has represented his Schizodus Schlotheimi with a deep sinus in the pallial impression: this 
is directly contrary to what I should have expected. However, so many errors have been made in connexion 
with this character, that I may be excused suggesting to Dr. Geinitz the propriety of a rigorous re-examina- 
tion of the two specimens represented in the ‘Versteinerungen’ (pl. in, figs. 31, 32). My specimens, 
unfortunately, do not dispiay any muscular impressions, nor have I seen the pallial muscular sinus in 
Schizodus truncatus—the species typical of the genus. An entire pallial line is only displayed in one of my 
specimens of Schizodus obscurus—the one referred to in the text. It may be stated, however, that I have 
seen precisely the same character displayed on one of my casts of Schizodus carbonarius, which I have no 
hesitation in considering a congeneric species, both from its superficial ornamentation, and dental system. 
