No. 1. Descriptions of Bolca Fishes. By C. R. Eastman. 



There are two principal sources of information in regard to the 

 marine fish fauna of the Eocene period, leaving out of account the 

 minor evidence that is presented by detached hard parts, such as teeth 

 and other fragmentary remains. The first of these, which is at the 

 same time the most important and historically the most interesting, is 

 that furnished by the tolerably abundant skeletons occurring in the 

 fissile limestone of Monte Bolca and Monte Postale in northern Italy. 

 The other is that association of ichthyic remains which is known from 

 the nearly equivalent horizon of the London Clay. 



These two faunas fortunately supplement each other to a consider- 

 able extent, one of them making us acquainted with the large variety 

 of forms which flourished during the later Eocene, and the other sup- 

 plying us witli important anatomical details. For the conditions of 

 preservation in clay beds are obviously very different from those which 

 are peculiar to limestone. Calcareous sediments are more compact; 

 and where pressure and subsequent hardening occur, bodies which are 

 not absolutely rigid, like the skeletons of vertebrates, or even the outer 

 covering of chelonians and crocodilians, are liable to become compressed 

 and flattened out. Hence, as a general rule, the parts belonging to 

 either side of the body in fishes become squeezed together and con- 

 fused when preserved in limestone, and the pliant head-bones become 

 more or less distorted and displaced. This is almost invariably the case 

 with the fishes from Monte Bolca, and for a correct understanding of 

 the cranial osteology we must turn to the uucrushed skulls from 

 Sheppey and elsewhere. 



The London Clay fauna,^ however, is not nearly so rich as the Italian, 

 either in point of numbers or variety ; and it is accordingly the latter 

 which provides us with the principal data for comparing the ichthyic 

 representation of Eocene and modern times. Comparisons of this 

 nature and of detailed structural modifications are of the very greatest 

 importance, since by their means we are able to trace the direction and 



1 Agassiz, L., Report on the Fossil Fislies of the London Clay (Rept. Brit. Assoc. 

 Adv. Sci. pp. 279-310, 1845). 



VOL. XLVI. — NO. 1 1 



