30 
SELACHn. 
Hyhocladodns plicatilis, St. John & Worthen, iom. eit. p. 286, 
pi. V. fig. 9. — Upper Burlington Limestone ; Iowa. 
ffybocladodus tenuicostatus, St. John & Worthen, tom. eit. p. 286, 
pi. V. fig. 10. — Keokuk Limestone ; Iowa, Illinois. 
Order II. SELACHII. 
Endoskeletal cartilage, as a rule, only superficially calcified. 
Notochord (except in a few early tj-pes) alwa}’S more or less con- 
stricted in the adult. Neural and h®mal arches and spines stout, 
with intercalary cartilages in the more specialized forms. Pectoral 
fins without segmented axis. Axial cartilages of the hind limb 
prolonged into a clasper in the male. 
Suborder I. TECTOSPONDYLI. 
Vertebrae, when fully developed, having the concentric calcified 
laminae predominating over the radiating laminae (tectospondylie, 
Hasso). Specialization resulting in a depression of the body, and 
an enlargement of the pectoral fins ; spiracles, of large size, retained 
in the most specialized forms. Anal fin absent. 
Family SPINACIDiE. 
Body round or trihedral, and very slightly depressed. Mouth 
gently arched ; snout obtuse. Pectoral fins not notched at their 
origin and not produced forward ; gill-slits small, lateral, often in 
the line of the pectorals, often half below. Spiracles large, behind 
the eye. 
Genus CENTRIPIA, Cuvier. 
[Kegne Animal, vol. ii. 1817, p. 130.] 
Syn. O.rynotus, Rafinesque Schmalz, Ind. Ittiologia Siciliana, 1810, 
p. 60 (incomplete definition). 
Powerful dorsal fin-spines present. Trunk rather elevated, trihe- 
dral, with a fold of skin extending along each side of the ventral 
surface. Teeth of the lower jaw erect, triangular, finely serrated ; 
those of the upper slender, conical, forming a group in front of 
the jaw. 
The following extinct species is founded upon detached teeth from 
the Pliocene of Orciano, Tuscany, described as scarcely distinguish- 
able from those of the living O. salviani of the Mediterranean. The 
figures show them to he remarkably similar to the lower teeth of 
Scymnus. 
