18 Memoirs of the Indian Museum. [VoL. IV, 
it comprises a large number of species inhabiting all the warm and temperate waters of 
the globe. Comparison of the species from the Atlantic, Mediterranean and W. Coast 
of America with those from the Indo-pacific seems to show that evolution has on the 
whole taken a distinct and separate course in the latter area. In the Indo-pacific a 
single species, Sguilla mantoidea, is the sole representative of the section to which the 
vast majority of Atlantic, Mediterranean and W. American forms belong—a section 
which includes all the common species in those localities.' On the other hand the 
Indo-pacific section that surpasses all others in point of individual abundance and 
comprises some fourteen” species, is represented in the other regions only by the West 
Indian Sguilla alba. The ‘ Chloridella’ group, moreover, is found only in the Indo- 
pacific. 
One species only, Sguilla armata, an isolated form wholly réstricted to southern 
latitudes, is common to both areas. 
A peculiar structural feature of the genus and one that, hitherto, has largely 
escaped attention, is the complete suppression of the mandibular palp in certain species. 
Hilgendorf first noticed its absence in the form which he called Pterygosquilla laticauda 
and, more recently, Jurich has made a similar discovery in the case of Squilla lepto- 
squilla. Among the species which I have been able to examine eight show no trace of 
this appendage :— 
Squilla gibba, Nobili. Squilla scorpio, Latreille. 
Squilla armata, Milne-Edwards. Squilla laevis, Hess. 
Squilla leptosquilla, Brooks. Squilla hieroglyphica, Kemp. 
Squilla tenuispinis, Wood-Mason. Squilla costata, De Haan. 
Squilla leptosquilla and S. tenuispinis are closely allied forms and the same is the 
case with S. Jaevis and S. hieroglyphica. 
In Decapoda the structure of the mandibular palp, its presence or its absence, is 
recognized as a valuable clue to the affinities of the various species and in many cases 
it affords almost the sole distinction between genera. It is clear that its importance 
is far less in the Squillidae. Sguilla gibba by many evident external characters takes 
its place in the ‘ Chlovidella ’. section of the genus, though it is perhaps a somewhat 
aberrant member thereof ; S. scorpio finds distant allies in S. data and S. gilesi; S. 
laevis and S. hieroglyphica seem to form a small subsection of the S. nepa and quingue- 
dentata group, while S. costata by the number of carinae on its abdomen appears to be 
related to S. muilticarinata. Squilla armata seems to be an isolated species and the 
same is the case with S. /eptosquilla and tenwispinis, which are restricted to deep water. 
It seems then that the palpless species found in the Indo-pacific fall into six sep- 
arate classes, which show no trace of common ancestry distinct from that of the other 
species of the genus, and the inference that the palp has been suppressed on no less 
than six separate occasions in the ontogeny of the Indo-pacifie species can scarcely 
be avoided. 
! Such as S mantis, empusa, dubia, prasinolineata and panamensts. 
S. laevis, quinquedentata, nepa, oratoria, stridulans, etc. 
