46 THE VOYAGE OF H.M.S. CHALLENGER. 



are only capable of being detected where they are largest and most important, such 

 as at the extremity of the ecphysis of the gnathopoda, and at the posterior margin of the 

 telson. 



Comparing the brephalos with that of Homarus, we find that it has the antenna; 

 further advanced in development, that there is no ecphysis attached to the first and 

 second pairs of pereiopoda, and that all the pleopoda are well advanced in development, 

 whereas in Homarus the pleopoda are all wanting. An ecphysis is attached to the basis 

 of the several pairs of pereipoda, and the flagella of both pairs of antenna; are in a 

 rudimentary condition. 



The carapace in both is small, covering only the pereion. 



We thus perceive that although the resemblance between them is great, particularly 

 in relation to the cephalon and pereion, a difference exists in the more advanced condi- 

 tion of the brephalos of Eieonaxius at the period when it quits the ovum, as compared 

 with the young of Homarus. 



In each of these genera the ova are extremely large, and few in number. 



Family ThauuastochelidjE. 



The carapace is ovate and smooth, and projects to an anteriorly-flattened point or 

 rostrum. The first pair of antenna; has two long flagella, and the second has a well- 

 formed scaphocerite. First pair of pereiopoda is chelate, large, having a long slender 

 dactylos and pollex ; subequal, somewhat unsymmetrical. Second pair chelate, symme- 

 trical, subequal, small. Pleon has the coxal plates well defined. Rhipidura having the 

 outer plates much larger than the inner. Branchial apparatus having a mastigobranchia 

 attached to all the appendages of the pereion, except the first pair of gnathopoda, where 

 it is rudimentary, and the posterior pair of pereiopoda ; five podobranchise, ten arthro- 

 branchia;, and four pleurobranchise on each side. 



The genera in this family are Thaumastochcles and Callocaris, 



Tha a mastocheles, Wood-Mason. 



Astacus, Sulim, Trans. Linn. Soc. Lond., ser. 2, vol. i. p. 48. 

 Thaumastocheles, Wood-Mason, Proc. Asiat. Soc. Bengal, p. 181, 1874. 



Carapace less than half the length of the animal, dorsally flattened and anteriorly 

 produced to a rostrum, divided by a moderately deep cervical sulcus ; near the centre the 

 lateral walls are depressed and the posterior margin is secured in its position by a strong- 

 blunt process (pleocleis) attached to the lateral portion of the first joint of the pleon. 



The pleon is longer than the carapace, and each somite increases in width posteriorly 

 to the fifth and then decreases. 



The ophthalmopoda are absent or obsolete. 



