680 MR JAMES MURRAY ON 
(13)—in which there are extra plates, the homologues of which are difficult to trace. 
The third median is often lacking, or it may be united to the lumbar; the second 
median also is sometimes absent. 
Owing to these variations, it is judged better to give under each species a formula 
setting forth the number and arrangement of the plates, and of the various processes 
(setae, spines, knobs) which they bear. When the homologous plates are recognisable, 
they will be numbered as above. 
Ricuters (9) divides the body into six principal segments :—I. (=head plate); 
IL. (=shoulder plate); Il. (=first pair); IV. (=second pair); V. (= lumbar plate) ; 
VI. (=twil-piece or anal plate). 
Processes.—All the species have processes of some sort; many have numerous setz, 
spines, or short knobs. Six sete are invariably present, viz., the four near the mouth, 
and a pair behind the head. Besides these, there are usually some dorsal and lateral 
hairs or spines. The dorsal processes arise (with the single exception of E. gladzator) 
from the posterior margins of the plates which bear them. The lateral processes also 
spring from the posterior margin, at the ventral limit of the plates (postero-ventral 
angle). They are regarded by Ricnrers as arising independently of the two plates 
between which they are found; but they seem to me to be always more intimately 
connected with the anterior of the two, and are often as rigidly joined to it as are the 
dorsal processes, remaining attached to it after the skin has been cast, and the softer 
integument between the plates decayed away. 
Following the practice of PLarr (5) and subsequent writers, the four short setze of 
the face are disregarded (as being invariable) and only the longer head pair reckoned 
among the lateral sete. It is understood that all processes are paired, and rise from 
the posterior edges of the plates (except the median spine of . gladiator). 
Dorsal processes are rarely found on any but the paired plates ; lateral processes may 
be on any or all of the plates which extend over the sides. 
Ricurers (12) distinguishes the lateral processes by the letters a, b, c, d,e; «=the 
head seta, b springs from the shoulder plate, ¢ from the first pair, @ from the second 
pair, e from the cut separating the tail-piece or anal plate from the lumbar plate; ¢ 
might as readily be reckoned dorsal as lateral, as when it is a spine it often rises some 
distance up the back. The positions of the various processes have been usually 
indicated by their relations to the four legs; they can be more accurately located by 
reference to the plates. The lateral processes, b, ¢, d, e, are over the four legs 
respectively. 
Texture of Skin.—The plates may be quite smooth (Z. islandieus), but are usually 
covered with larger or smaller granules, which may be of equal size and uniformly 
distributed, or irregular both in size and spacing. Some appear to have perforations in 
place of granules, or show other peculiarities, which will be noticed in the detailed 
descriptions. The whole of the skin, as well as the plates, is sometimes finely granular, 
the proximal part of the legs in some species coarsely so. 
