BRITISH FOSSILS. 31 



branches (endopodite and exopodite). In the former a long- 

 proximal and a short distal joint are distinguishable. The 

 proximal joint is united, for a considerable part of its length, to its 

 fellow on the opposite side by a flattened plate, continuous above with 

 the sternum, and thickened by a spatulate ridge in the middle line. 



The distal joint has the inner half of its terminal margin more 

 produced than the outer. The former bears a shorter (endopodite), 

 and the latter a longer, three-jointed branch (exopodite). The 

 terminal articulation is the largest, and is provided with long and 

 strong setse. 



The inner edges of the basal joint of the protopodite of the 

 fifth pairs of these feet are beset from their distal extremities to the 

 uniting plate with a series of short, but strong and pointed teeth. 



On considering the structure of Calanus with reference to the 

 general plan of the Crustacea, I may observe, in the first place, 

 that of the twenty typical somites, only eighteen, at most, appear 

 to be represented. We may reckon one, for the eyes ; two, for the 

 antennules and antennae ; four, for the four pairs of post-oral 

 appendages ; and five, for the swimming feet, making, in all, eleven, 

 provided with moveable appendages. Then, if the first segment 

 of the abdomen be really formed, as there is much reason to believe 

 it is, by the coalescence of two somites, and if the two caudal 

 styles really represent another, we shall have six somites in the 

 abdomen, making up the sum total to eighteen. 



This number cannot by any means be made out in all Copepoda, 

 but I know of no members of the order which exceed it. 

 Different opinions have been entertained with respect to the 

 grouping of these somites into cephalic, thoracic, and abdominal. 

 The eyes, antennulse, antennae, and the two anterior pairs of post- 

 oral limbs, and with them their somites, are indeed universally 

 admitted to belong to the head ; but the two following appen- 

 dages are considered by some to be maxilla3, by others to be 

 maxillse and maxillipedes ; by yet others to be maxillipedes and 

 anterior thoracic limbs. 



The simplest and most natural view appears to me to be, to 

 regard the third pair of post-oral appendages as the homologues of 

 the second maxillse or last cephalic appendages, while the fourth 

 post-oral appendages are thoracic members. In this case there will 

 be six pairs of thoracic appendages, as in the Cirripedia. If we 

 inquire what has become of the two thoracic somits which are unre- 

 presented by appendages, three probabilities present themselves : 

 1. They have remained undeveloped at the anterior end of the 



