SEIACHIT. 
30 
ITt/bocladodus pit cat dis, St. John & AVorthen, tom. cit. p. 286, 
pi. v. fig. 9.—Upper Burlington Limestone : Iowa. 
Hifbodadodus tenmcostafus, St. John & Worthen, tom. cit. p. 286, 
pi. v. tig. 10.—Keokuk Limestone; Iowa, Illinois. 
£.U <T Jlwt- sjh-n-i* « 
' r Order II. SELACHII. 
^ fiJLj' ® n ^ os keletal cartilage, as a rule, only superficially calcified. 
* r ///V ^ otoc ^ or ^ (except in a few early types) always more or less con- 
r strieted in the adult. Neural and haemal arches and spines stout, 
/ with intercalary cartilages in the more specialized forms. Pectoral 
'Zi / j-'6G &is \jjfhout segmented axis. Axial cartilages of the hind limb 
rT “/ d into a clasper in the male. 
fa****} J k b -v 
p \Z*~ Suborder I, TECTOSPONDYLL 
\ Vertebrae, when fully developed, having the concentric calcified 
laminae predominating over the radiating laminae (tectospondylic 
Vi* 
^ -^ asse )* Specialization resulting in a depression of the body, and 
an enlargement of the pectoral tins ; spiracles, of large size, retained 
in the most specialized forms. Anal tin absent. 
Family SPINACID2E. 
Body round or trihedral, and very slightly depressed. Month 
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 halt below. Spiracles large, behind 
the eye. 
Genus CENTRINA, Cuvier. 
[Kegne Animal, vol. ii. 1817, p. 130.] 
Syn, Oxi/notus, Kafinesque Sehmalz, Ind. Ittiologia Siciliana, 1S10, 
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 0. set 1 vicini of the Mediterranean. The 
figures show them to be remarkably similar to the lower teeth of 
Scumnus. 
