IITTKODCCTION. 
XV 
are those from the Upper Eocene of Monte Bolca ' and Monte 
Postale ’ in North Italy, described by Heckel, Molin, A. de Zigno, 
and others ; and from the Green Eiver Shales of Wyoming, U.S.A., 
described by Prof. Cope 
Stsopsis or Pal^osiological Eesclts. 
Summarizing the general results of these discoveries and investi- 
gations, aud adding much that is new, suggested hy a study of the 
British-Museum Collection, the main points of biological significance 
may be briefly enumerated as follows : — 
Cartilaye. 
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 Pleuramnihtis (p. 2) 
and Hyhodopsis (p. 240), display the well-known superficial crust 
of polygonal calcified tesseraj 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 Chondrenehelys (p. 15) and the late 
Palffiozoic Pleuracanthus 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 * 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 Chondrenehelys (p. 15). 
Among later Selachians there are a few instances in which a 
• Bhinohatvs prinusvus (p. 82), Flafyrhina holcemis (p. 459), Myliobaiis 
gazolai (p. 124), Torpedo egertoni (p. 90), Torpedo gigan/ea (p. 90), species of 
Trygon (p. 153), Mesifeia emiluB (p. 346), and Protogaleus cuvicri (p. 4.37). 
’ Trygonorhina dezignii (p. 83) and Urohphus prineeps (p. l.'id). 
’ Xipholrygoa acufidens (p. 154). 
’ Bull. Mns. Comp. Zoology Harvard Coll. vol. xii. no. 1 (1886). p. 29. 
