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



cervical fossa than anterior to it ; beyond this is another that appears to commence in 

 the hepatic tooth, which varies in importance in different species, and continues to the 

 posterior margin of the carapace ; a fourth ridge on each side commences in the post- 

 antennal tooth, is interrupted at the cervical fossa, and then continued to the posterior 

 margin of the carapace ; again, beyond this, is another ridge that is generally less 

 conspicuous, and sometimes appears to divide and traverse the carapace from the anterior 

 to the posterior margin, whilst a strong submarginal ridge, commencing at the antennal 

 tooth, traverses a line within, corresponding with the margin of the carapace, and 

 continues from the frontal and approaches in the median line at the posterior margin. 



The pleon is generally tuberculose even in the smoother forms, the texture of the 

 somites is universally rigid, and the dorsal surface is elevated in the median line into a 

 strong but broken ridge, the anterior extremity of which is produced into an anteriorly 

 projecting cusp which rests in the extended position of the animal on a smooth 

 depression in the carapace. A similarly formed tooth also exists on each side and 

 similarly rests against the carapace in a line corresponding with the second dorsal ridge. 

 The lateral margins are produced to strongly projecting teeth that vary in number and 

 importance, there being only one on the first, two or three on the second, two on the 

 third, fourth, and fifth, and one on the sixth somite. The four anterior somites are 

 united by a small hinge joint, consisting of a small protuberance at the postero-lateral 

 margin, which rests in a corresponding hollow of the anterior margin of the succeeding 

 somite. 



The posterior articulations of the three last somites differ from the preceding. The 

 three anterior articulate by a process projecting from the posterior margin, and which 

 rotates in a cup on the anterior margin of the next succeeding somite. The three 

 posterior somites possess a button-shaped protuberance projecting from the anterior 

 margin, which is lodged in a hollow produced by a curved process projecting from the 

 posterior mai'gin of the one preceding. 



A slight modification exists in the last somite ; the curved process or articulation 

 is more ovate, and the telson has the anterior process implanted within it ; so that by a 

 slight contraction of the flexor muscles, the telson is forced into a fixed position, and 

 is only released when these muscles are relaxed. 



The telson is a long bayonet-shaped organ ; it is transversely quadrate, each angle 

 forming a longitudinal rigid and elevated carina which converges to a sharp point 

 that is slightly curved upwards. It is generally longer than the lateral plates of the 

 rhipidura, which are membranous, soft and flexible. On the anterior portion of the 

 dorsal grove of the telson is a strong cusp that generally underlies the projecting tooth 

 of the sixth somite, which, when the animal is extended, presses against its posterior 

 surface and strengthens it in its position. The telson in this genus is undoubtedly 

 an offensive weapon, and a very powerful one when the animal wills to strike, but should 



