244. TRIONYCHID As. 
present, I feel reluctant to admit species founded upon the modifi- 
cation (and a modification assumed but in a comparatively late stage 
of existence) of a single organ, unaccompanied by any but correlative 
differences ; and I maintain under the same species carnivorous as 
well as conchifragous individuals. As before stated, similar varia- 
tion is met with at distant points of the globe, and I have no doubt 
that, as our knowledge progresses, other instances of this supposed 
dimorphism will be brought to light. 
Synopsis of the Species. 
I. Costal plates eight pairs normally; young with numerous 
longitudinal dorsal dermal ridges or series of tubercles.— 
Old World. 
A. All the costal plates separated 
on the median line by the 
neural plates ; head very large ; 
postorbital arch extremely nar- 
row, forming a keel on the 
OUtEr SUMLACEN lay. /c:tte.~ ellaieie 1. subplanus, p. 246. 
B. Last pair of costal plates in contact on the median line ; 
postorbital arch expanded on the outer surface. 
1. Two neural plates between the first pair of costals ; 
alveolar surface of lower jaw without a strong longi- 
tudinal ridge at the symphysis. 
Inner edge of alveolar surface of man- 
dible strongly raised, forming a sharp 
ridge, which sends off a short perpen- 
dicular process at the symphysis ; inter- 
orbital space, in the adult skull, nar- 
rower than the external nasal opening ; 
head with oblique black lines, con- 
verging forwards; no ocellar spots .. 2. gangeticus, p. 248. 
Inner edge of alveolar surface of mandible 
not or but very slightly raised, sym- 
physis flat or with a longitudinal 
furrow; interorbital space narrower 
than the external nasal opening ; head 
with oblique black lines, converging 
forwards ; young with small ocellar 
dorsalispots.tc a ariys ome ehdees oie = 3. leithu, p. 249. 
Inner edge of alveolar surface of man- 
dible not or but very slightly raised, 
symphysis flat or with a longitudinal 
furrow ; interorbital space, in the adult 
skull, broader than the external nasal 
opening ; head black marbled, with a 
large yellow spot behind each eye; 
young with large ocellar dorsal spots. 4. hurum, p. 249. 
