188 PERMIAN FOSSILS. 



(vide PI. XV, fig. 23) the adductor muscular impressions {a, b), pedal or visceral ditto 

 {d, e), and entire pallial line (c), are often instructively displayed.i 



The cartilage of one of my specimens of ScJdzodus axiniformis 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 Tric/onia, 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 Myophoria, 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 Trigonia, 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. Trigonia is generally ornamented with ribs, tubercles, and varices, 

 frequently combined : Myopltoria is usually characterised with a variable number 

 of longitudinal ribs : and ScJn%odus is, in general, simply marked with fine raised 

 concentric lines. 



By means of its relation to Myophoria and Schizodus, the singular genus Trigonia 

 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. Geiuitz 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 heen 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 'Versteineruugen' (pi. iii, figs. 31, 32). My specimens, 

 unfortunately, do not display 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. 



