372 CRETACEOUS PELECYPODA 



The shells are stated to be inequivalye^ though this character appears to be 

 barely noticeable in some of the figured specimens. The posterior part of the 

 shell is covered with a paleaceous cuticule, but this also occurs in other species 

 of Mytili and Modiolce, It seems hardly appropriate to create ,a new name for 

 these forms as distinct from Mytilus. 



'Fossil Mytili occur rarely in palaeozoic strata and appear gradually to increase 

 from that time up to the present. They are now less numerous than the 

 Modiolce, and the same also seems to haye been the case already during the 

 mesozoic epoch. - 



List o;f gretaceous species of Mytilid^,. 



The correct generic determination of the various cretaceous species is accompanied witli 

 considerable difficulties, although/ as a rule, fossil MYTiLlDyE are found well preserved; but this does 

 not apply to the internal characters^ and genera like Breissena, or Congeria, and Septifer can satis- 

 factorily be separated from their allies merely by the study of the internal structure of the shell. 



The sub-family dreisseninje is represented in the cretaceous deposits only by a few species 

 which are with great probability referable to Septifer. Of the crenellin^ a few species of 

 Modiolaria and Crenella (including Mi/oparo or Btalagmmm) occur. Most of the species known 

 belong, however, to Lithodomus, Modiola, and MytUus which, including a few less important sub- 

 genera, form the third sub-family. The distinction of the fossil species of the three last named 

 genera is, even from mere external appearance, not more difficult than that of recent specieS; and 

 it is approved of by most palaeontologists in opposition to Desh ayes, who unites them under the 

 single Linnean name Mytihis^ 



In order to avoid numerous repetitions of references to literature, I shall again follow in the 

 enumeration of the species Pictet and Campiche who class our shells in two genera, Mytilus and 

 Lithodomus (Mat. p. 1. Pal. Suisse, ivme ser., ^^^ part., 1867, p. 50a et sec^.). Later additions are 

 eomparatively few. 



Mytilus, Modiola, Septifer, and Crenella. 



\,~Myt. lanceolatus,^ Sow. (? including edentuhis, tridens, pr^elongus, and inceqiiimlvm of the- 

 same author). 



2-4.— Jfj/^. Couloni, Marc, Miclaillensis, P. and C, SanctcB-cmcis, P. and C, may with equal 

 probability belong to Septifer or to Mytilus, 



5. — Myt. Cuvieri, Math., is a Modiola, under which genus it was originally described. 



^-l.—Myt. Bubisiensis, P. and C, Yoironensis, P. and Loriol. 



^.—Myt. Morrisii, Sharpe, has probably to be referred to BracJiydontes, a sub-genus of Modiola, 



^-l^.'-Myt suhsimplex and Bittoni, d'0ih.,wqualisy Sow., Montmollmi, P. and C, Carteroni^ 

 d^Orb., are Modiola. 



■U.—Myt, Salevensis, Lorial, (Favre, fos& de Mont. Saleve,pl. C, figs. 20-21, p. 385,) belongs 

 to BracJiydontes. 



Ih.—Myt. Besoriamis, Loriol, (ibidem pi. C, figs. 18-19,) resembles more a Myoconcha than a'' 

 Modiola. 



le.^Modiola hella. Sow., is probably a Crenella, 



11 -U.-^Modiola Matronensis, cVOvh., Mod. Gillieroni, P. and C, Mod. undnlata, d^Orb.^ 

 Mod, pulc]lerr^ma,l^^QQm.., i^. 2. Modiolaria), Mod. mgma,^^m.. - • 



I'h.-^Myt, densesulcatus, d'Orb., a ribbed species, but not further characterized. 



* Mem. Cour. Acad. Belgique, xxxiv, 1870; Briart et Cornet,. Foss. de BracqLuegmes, p, 5^. 



