310 CEETACEOUS PELECYPODA 



3. Neoschizodus, Giebel, 1856. (Verst. des Muschelk. von Lieskau, p. 40, 

 in Abhand. Nat. Verein, Halle). Shell similar to ScUzodus, hinge of the left 

 valve with a large posterior sub-terminal cardinal tooth, posteriorly prolonged, 

 parallel to the fulcra which are distinct from the margin itself, and with an 

 anterior terminal cardinal ; right valve wdth a marginal elongated posterior and a 

 sub-terminal shorter but thicker anterior cardinal tooth; type, N. IcEvigatus, Giebel, 

 from triassic beds near Lieskau (Germany). According to the characteristic taken 

 from the figure of the type-, and another, species, Giebel' s NeoscUzodus must be 

 considered as distinct from ScUzodm, as well as from MyopJioria, with which it 

 has lately been identified by several authors, for unless the figures have been 

 proved to be incorrect, these identifications cannot be accepted. 



4. Myophoria, Bronn, 1835,* fvide Leth. geog., 2nd edit., II, p. 66; E^ichter 

 in Zeitsch. deutsch. geol. Gesellsch,, 1869, xxi, p. M4). Shell similar to ScMzodus, 

 smooth or concentrically striated, or partially radiately ribbed ; hinge in the left 

 valve with three diverging sub-equal cardinal teeth, the middle one being the 

 thickest and sometimes bifid, while the outer ones are marginal ; the right valve has. 

 two teeth, both of which are marginal, and the posterior sometimes striated or denti- 

 culated ; the other teeth are rarely distinctly striated ; muscular impressions close to 

 the hinge-teeth, and generally margined on the internal side by a raised rib ; type, 

 M. vulgaris, Schloth. All the species as yet known are from triassic rocks, and 

 chiefly from the upper series of that formation. The hinge-teeth greatly resemble 

 in their arrangement those of Schizodus, while those of NeoscUzodus appear to be 

 closely allied to those of Cardita, as noted by Giebel. 



5. Trigonia, Brug., 1789, fLyriodon apud Bronn). Shell oblong, inequi- 

 lateral, with the beaks sub-anterior, rounded in front, more or less produced 

 and obliquely truncate posteriorly, pearly inside; ornamentation variable, with 

 concentric or radiating or sometimes divaricating ribs ; hinge with two diverging 

 teeth in the right valve and three in the left, the median large, sub-trigonal, 

 internally indented, the outer comparatively very small, thin, similar to those in 

 Schizodus or Myophoria ; the teeth of the right valve are transversally ribbed on 

 both sides, those of the left only internally, where the right ones fit in ; type, 

 T, margaritacea, Lam. Possil species occur from the Lias upwards. 



Agassiz, in his '' Memoire sur les Trigonies" (Etudes m. fossiles, 1, 1840,) pro- 

 posed to group the numerous, chiefly fossil, species into several sections, merely to 

 facilitate the arrangement of the various forms. I shall briefly allude to them as 

 such convenient sections. The transitions from one group to the other are so 

 gradual and varied that no strictly classificatory value can be attached to these, 

 at least hardly equivalent to what may be called sub-genera. There also appear to 

 be too many sub-divisions ; they could be with advantage reduced to five or six, 

 and would thus admit of a better defiuition. 



a. Scaphoidea (including Clavellatce and ScabrceJ. Shell scaphoid, truncate, 

 or slightly convex in front, with sub-terminal and somewhat recurved beaks, poste- 

 riorly attenuating by a very easy curve, the upper edge being somewhat concave ; 



* The year 1829 is quoted by Bronn himself " in litteris." 



