i8 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, Squilla 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. 1 On the other hand the 

 Indo-pacific section that surpasses all others in point of individual abundance and 

 comprises some fourteen 2 species, is represented in the other regions only by the West 

 Indian Squilla alba. The ' Chloridella ' group, moreover, is found only in the Indo- 

 pacific. 



One species only, Squilla armata, an isolated form wholly restricted 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, I^atreille. 



Squilla armata, Milne-Edwards. Squilla laevis, Hess. 



Squilla leptosquilla , Brooks. Squilla hieroglyphica, Kemp. 



Squilla tenuispinis , Wood-Mason. Squilla costata, De Haan. 



Squilla leptosquilla and 5. tenuispinis are closely allied forms and the same is the 

 case with 5. laevis and 5. 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. Squilla gibba by many evident external characters takes 

 its place in the ' Chloridella ' section of the genus, though it is perhaps a somewhat 

 aberrant member thereof ; 5. scorpio finds distant allies in 5. lata and 5. gilesi ; S. 

 laevis and 5. hieroglyphica seem to form a small subsection of the 5. nepa and quinque- 

 dentata group, while 5. costata by the number of carinae on its abdomen appears to be 

 related to 5. multicarinata . Squilla armata seems to be an isolated species and the 

 same is the case with 5. leptosquilla and tenuispinis, 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-pacific species can scarcely 

 be. avoided. 



1 Such as S mantis, empusa, dubia, prasinolineata and panamensis. 



2 5. laevis, quinqiiedentata, nepa, oratoria, stridulans. etc. 



