HORNES TERTIAEY MOLLUSC A. 17 



are in the proportion of 1 to 4, or of 1 to 5, and (2) the curves 

 which indicate the sutures of the charahers, and which present a re- 

 markable analogy with those shown by the sutures of the chambers 

 in certain Goniatites. 



M. Barrande has not associated this American species with the 

 Orthoceratites which compose Group 2, already described, the his- 

 toric type of which is 0. triangulare — because, although there exists 

 between these different forms a certain analogy, they appear to be 

 sufficiently distinguished by the following dilfferences : — 



(1) The triangular section of the Orthoceratites of Group 2 is nearly 

 in the form of an equilateral triangle, with its three angles slightly 

 unequal ; but in G, anceps the section shows a very flattened triangle, 

 of which the lateral angles are veiy sharp, and very difl^erent from 

 the angle at the summit. 



(2) The undulations of the suture of the chambers are verj'- pro- 

 nounced in G. anceps. On the contrary nearly all the forms of 

 Group 2 show a simple suture. 



(3) All the species of Group 2 are characterized by the presence 

 of an organic deposit, which fills up their siphon, in the form of ra- 

 diating lamellae. Upon the specimens of Goniocems yet observed 

 no trace of a similar deposit has been discovered. Besides these dif- 

 ferences founded upon the characters of the forms, there exists a 

 remarkable contrast with regard to the period of their appearance. 

 The species which belong to Group 2 existed during the last phase of 

 the Third Silurian fauna in Bohemia, and elsewhere in the Devonian 

 fauna. On the contrary, G. anceps belongs exclusively to the lime- 

 stone of the Black-River group, which is equivalent to the middle of 

 the horizon occupied by the second fauna in America, and the part 

 where this fauna did not acquire its full development, [A. S.] 



The Fossil Mollusca. of the Tertiary Basin of Vienna. Vol. II. 

 Bivalves. Nos. 17 & 18. By Dr. Moritz Hornes. 



[Die fossile Mollusken des Tertiarbeckens von Wien. Yon Der Moritz Homes. 

 Band II. Bivalven. Lief. 17 & 18. Proc. Imp. Geol. Inst. Vienna. Dec. 3rd, 1867.] 

 The Mytilacese are represented in the Vienna basin by 9 species of 

 Modiola, 1 species of Litliodomus, 5 species of Mytilus, 3 species of 

 Congeria, and two species of Pinna. 



Of Modiola some species are found in the marls and sands of the 

 Leithakalli, while others {e. g. M. marginata, M. Volhynica, Eichw., 

 and M. Letochce, Homes) occur exclusively in the Sarmatic beds. 



The only species of Lithodomus which has been properly determined 

 is L. Avitensis, Mayer ; but some small nuclei from the Leithakalk 

 may belong to L. suhcordatus, D'Orb. 



Among the Mytili, M. Haidingeri is remarkable for its abundance 

 in certain strata, and for its peculiar ornamentation. Near Gaudem- 

 dorf it is associated with a species the brown epidermis of which is 

 constantly preserved. This has been regarded as a young form of 

 M. Haidingeri, but is determined by the author to be a distinct 

 species — M. fuscus. M. oUitus, Mich., and M. superlus, Horn., are 

 remarkable for the elegance of their ornamentation. 



