INTRODUCTION. 
XV 
are those from the Upper Eocene of Monte Bolca 1 and Monte 
Postale 2 in North Italy, described by Heckel, Molin, A. de Zigno, 
and others ; and from the Green River Shales of Wyoming, U.S.A., 
described by Prof. Cope 3 . 
Synopsis op Paleontological Results. 
Summarizing the general results of these discoveries and investi¬ 
gations, and adding much that is new, suggested by a study of the 
British-Museum Collection, the main points of biological significance 
may be briefly enumerated as follows :— 
Cartilage. 
Even among Elasmobranchs so early as those of the Lower Car¬ 
boniferous, the cartilages exhibit a considerable amount of calcifi¬ 
cation. A few Carboniferous genera, such as Fleur acanthus (p. 2) 
and Hybodopsis (p. 240), display the well-known superficial crust 
of polygonal calcified tesserae upon the cartilage ; but a considerable 
number of the older skeletons seem to exhibit a more penetrating 
and irregular distribution of the centres of calcification than is 
common among types of a later date. It is also interesting to note 
that in the Lower Carboniferous ChondrencJielys (p. 15) and the late 
Palaeozoic Fleur acanthus the slender cartilages present a curious 
concretionary arrangement of the calcareous salts, imparting to 
them a beaded appearance. 
Head and Visceral Arches. 
Concerning the cranium itself in extinct Elasmobranchs, there 
is at present very little information. Mr. Garman 4 has already 
pointed out that there is much reason to suspect a misconception in 
Prof. Cope’s statements as to the presence of distinct bony elements 
in the skull of the Ichthyotomi; and, if so, the only other divergence 
that has yet been noted between the cranium of these early Elas¬ 
mobranchs and the modern type is the possible presence of a basal 
membrane-bone (parasphenoid) in ChondrencJielys (p. 15). 
Among later Selachians there are a few instances in which a 
1 EJiinobatus primcevus (p. 82), Flatyrhina bolcensis (p. 459), Myliobatis 
gazolai (p. 124), Torpedo egertoni (p. 90), Torpedo gigantea (p. 90), species of 
Trygon (p. 153), Mesiteia emilioe (p. 346), and Protogaleus cuvieri (p. 437). 
2 Trygovorhina dezignii (p. 83) and Urolophus princeps (p. 154). 
3 Xiphotrygon acutidens (p. 154). 
4 Bull. Mus. Comp. Zoology Harvard Coll. vol. xii. no. 1 (1885). p. 29. 
