XCvi PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [May 1 896 



Ceratiocaris pajoilio, C. stygia, and C. Halliana, the carapace 

 is large and is composed, as in the living Nehalia, of a re- 

 duplication or fold of the dorsal integument of the head-segments, 

 which stretches backwards and forms a separate cover or shield, 

 not only to the cephalic and thoracic segments but even to some of 

 the abdominal ones as well. 



In some of the later forms, more particularly in species like 

 Ceratiocaris scorpioides and C. elongatus, described and figured by 

 Mr. B. N. Peach, F.E.S., from the Lower Carboniferous rocks of 

 Eskdale, the carapace is quite small and only about one-fourth the 

 length of the body, leaving about 10 segments of the slender 

 thorax and abdomen exposed to view. 



This is precisely what one sees in the living forms of the genera 

 Cuma, Bodortia, Diastylis, etc., and one is led to the conclusion 

 that we have here, in all probability, a passage upward from the 

 more ancient Phyllocarid type (in which a larger expansion of the 

 head-shield and a larger number of post-cephalic segments exist), 

 to the more modern Cumacea (in which only a small dorsal shield is 

 developed, formed partly by connecting some of the anterior thoracic 

 terga with those of the head), and thus gradually leading up to the 

 Decapoda (in which all the thoracic terga unite with the head to 

 form a true cephalothoracic carapace as in Astacus and Eomarus). 



Podophthalma. — 1. The Stoma topopa — represented at the pre- 

 sent day by Squilla and five other genera — are of especial interest 

 to us as, they offer so many important points of difference in their 

 structure from other adult Crustacea. It is in this sub-order that 

 the typical number of 21 segments can always be distinguished. 

 The carapace is quite small, and only covers a part of the cephalo- 

 thorax, while the gills are carried exposed in tufts attached to the 

 abdominal swimming-feet. The second maxillipeds — usually small 

 mouth-organs — are developed in Squilla into a pair of large and 

 powerful raptorial claws, taking the place of the chelate limbs of 

 the common lobster and crab. 



Prom their extensive distribution over the seas of the globe, 

 their past life-history and high antiquity, the Stomatopoda justly 

 challenge the attention of the palaeontologist. 



They make their appearance as far back in geological time as 

 the Coal Measures * — one species, Necroscilla Wilsoni (H. Woodw.), 



1 There is a doubtful form (Amphipeltis paradoxus), Salter, occurring 

 in the Devonian of St. John, New Brunswick, which may belong to the 

 Stomatopoda. 



