FISHES CHAP. 
1 
ty 
aN 
structure in the different families, ranges from a uniform cover- 
ing of dermal denticles to a condition in which the denticles 
fuse to form anteriorly a highly characteristic tessellated or 
continuous dorsal shield, while posteriorly they become replaced 
by a nearly typical rhombic squamation. The tail is hetero- 
cercal. Paired fins of the ordinary piscine type are absent. In 
some Ostracodermi it seems probable that the gill-clefts opened 
into a common branchial chamber on each side, with a single 
external aperture, but in others they may have been ventral. 
The endoskeleton, jaws, dentition, and the nostrils are unknown. 
Order I. Heterostraci. 
The exoskeletal structures consist of dentine, or of a tissue 
resembling it, never of true bone. The orbits are marginal or 
lateral in position. With the ex- 
ception of a caudal fin there are 
no median fins. 
Fam. 1. Coelolepidae.'—Head 
and anterior portion of the body 
flattened and expanded, with promi- 
nent lappet-like postero - lateral 
lobes, which may represent continu- 
ous lateral fin-folds or a’ very 
primitive type of pectoral fin. 
Nothing is known of the mouth, 
but it must have been ventral, nor 
of the position of the orbits. 
Branchial apertures unknown, but 
transverse markings on each side 
of the anterior part of Zhelodus 
paget may be indications of a bran- 
chial apparatus. The exoskeleton 
Fig. 318.—Restored outline of Lan- “consists Of & Umitorm covermeaman 
arkia spinosa, in the position in : : 
which it oceurs asa fossil, the head hollow pointed spines, devoid of a 
being flattened and the tail twisted basal plate and open below (Lan- 
round so as to appear in profile. On ; ; ; 
each side a much enlarged dermal arkia) ; or of minute shagreen-like 
dencicle ise a oie 7 DEO Rie aee tubercles (7helodus). The tubercles 
or spines consist of dentine coated by ganoim. Of the only two 
known genera, 7helodus is a characteristic Upper Silurian genus 
1 Traquair, Zrans. Roy. Soc. Edinb. xxxix. 1899, pp. 595 and 828. 
