^^^ CRETACEOUS PELECYPODA 



3. Neoschizo(b(s, Giehel, 1856. (Verst. des Musclielk. von Licskau, p. 40, 

 in Abhand. Nat. Verein, Halle). Shell similar to Schizodus, liinge of the left 

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

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

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

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

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

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

 considered as distinct from Schizodus, as well as from Ilyophoria, 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. Miinphona, Bronn, 1835,* (vide Leth. geog., 2nd edit., II, p. 66; Richter 

 in Zeitsch. deutsch. geol. Gesellsch., 1869, xxi, p. 444). Shell similar to Schizodus, 

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

 M. mdfjaris, 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 Neoschizodus appear to be 

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



5. Trigonia, Brug., 1789, ( Lyriodon 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 Mijophoria ; 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. Fossil 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 Avhat 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 definition. 



a. Scaphoidea (including ClavellatcB and ScahrceJ. Shell scaplioid, 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 somcAvhat concave ; 



* The year 1829 is quoted by Bronn liimsclf " iu litteris." 



