372 CRETACEOUS PELECYPODA 



The sliells are stated to be inequivalve, 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 Ilijtili and Ilodiolcc. It seems hardly appropriate to create a new name for 

 these forms as distinct from Ilijtilus. 



Eossil My till occur rarely in palaeozoic strata and appear gradvially to increase 

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

 ModiolcB, and the same also seems to have been the case already during the 

 mesozoic epoch. 



List of cretaceous species of BIytilid^e, 



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

 considerable difficulties, although, as a rule, fossil Mytilw^ are found well preserved j but this does 

 not apply to the internal characters, and genera like Breissena, or Conr/eria, 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 crenellinje a few species of 

 Modiolar'ia and Crenella (including Mijoparo or Stalagmium) occur. Most of the species known 

 belong, however, to Lithodomus, Ifodiola, and Mi/tUus 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 Deshayes, who unites them under the 

 single Linnean name Mytilus. 



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, Mijtilus and 

 Lithodomus (Mat. p. 1. Pal. Suisse, ivme ser., S'"^ part., 1867, p. 506 et seq.). Later additions are 

 comparatively few. 



3Ii/tilus, MoJiola, Septifer, and Crenella. 



1. — Mi/t. lanceolalus* ^ovf. ij mcXyxiirag edentulus , tridens, prailongiis, anH incequivalvis of the 

 same author). 



2-4. — Myt. Cotiloni, Marc, Michaillensis, P. and C, Sanctee-crncis, P. and C, may with equal 

 probability belong to Septifer or to Mi/tilus. 



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



6-7. — 3Iyt. Buhisiensis, P. and C, Voiro7iensis, P. and Loriol. 



8. — M^t. Morrisii, Sharpe, has probably to be referred to Brachydontes, a sub-genus of Modiola. 



9-13. — Ili/t. subsimplex and Fittoni, d'Orb., cequalis, Sow., MontmoUiiii, P. and C, Carteroni, 

 d'Orb., are Modiola. 



14. — Myt. Salevensis, Loriol, (Favre, foss de Mont. Saleve,pl. C, figs. 20-21, p. 385,) belongs 

 to Brachydontes. 



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

 Modiola. 



16. — Modiola hello, Sow., is pi-obably a Crenella. 



17-21. — Modiola Malronensis, d'Orh., Mod. Gillieroni, P. and C, Mod. undulala, d'Orb., 

 Mod. pulcherrima, Rojm., (? a Modiolaria) , Mod. rugosa, Roem. 



22. — 3Iyt. densesulcatits, d'Orb., a ribbed species, but not further characterized. 



* Mc'in. Cour. Acail. Bdijique, xxxiv, 1S70; Briait et Cornet, Foss. do Bracquegnics, p. 52. 



