BRITISH FOSSILS. 33 



timate. It is a subchela such as is found in Amphipoda and 

 Stomapoda, not a chela like that of Podoptithalmia and 

 PcecilopodcL 



4. The plate, which corresponds with the conjoined epistoma 

 and labrum of decapod Crustacea, is very large in all the Cope- 

 poda I have examined. Its form and proportions in Calanus, 

 Corycceus, and Pontella are shown in (Plate XVI. figs. 2, 9, 11). In 

 the last-named genus it exhibits, as Milne Edwards has pointed out 

 in his great work on the Crustacea, the remarkable peculiarity of 

 being divided into three lobes, of which the middle is the smallest, 

 and constitutes a kind of tongue-like projection (fig. 11). 



5. The metastoma of Calanus, as has been stated above, is 

 excavated anteriorly by so deep and wide an emargination, that it 

 almost appears to consist of distinct lobes. In Pontella, on the 

 other hand, the metastoma is a large flattened plate, whose ter- 

 minal emargination, though wide, is not deep (fig. ] 1). 



6. In Calanus, nine pairs of post-oral appendages have been 

 found, the greatest number possessed by any masticating copepod. 

 In Corycceus and Cyclops this number is reduced to eight, and of 

 these the last is occasionally so much atrophied as to be hardly 

 distinguishable. We have thus evidence of a certain tendency 

 towards a diminution of the number of thoracic appendages in this 

 order. 



The study of the development of the Copepoda shows that this 

 is no more than might be expected, when we take into consideration 

 how common a thing it is, for some form of a group to retain more 

 or fewer of the characters usually found only in the young of that 

 group ; and perhaps the only subject for astonishment is that adult 

 Copepoda, with still fewer limbs, have not yet been discovered. A 

 Copepod just hatched is, in fact, a very different creature from the 

 adult, as has been known since the observations of Jurine upon 

 Cyclops. 



Rathke, who has devoted particular attention to the development 

 of the embryo in this genus, maintains that of the three pairs of 

 locomotive appendages, with which alone, the young is endowed 

 when it leaves the egg, the two anterior eventually become the 

 antennules and antennae, while the posterior are neither the man- 

 dibles nor the first pair of maxillae, but the rudiment of the " mains" 

 of Jurine. The young Cyclops is in addition provided with a large 

 epistomo-labral plate. Its further changes consist chiefly in the 

 elongation of the body, and the gradual and successive acquisition, 

 of the thoracic members. 



[1.] c 



