154 DR. P. P. C. HOEK OX DICHELASPIS PELLUCIDA. 



On Dichelaspis pellucida, Darwin, from the scales of an Hydro- 

 phid. obtained at Mergui. By Dr. P. P. C. Hoek, Member 

 Royal Academy of Science of the Netherlands, Leiden. 

 (Communicated by Dr. J. Anderson, F.R.S., I\L.S.) 



[Eead 17th February, 1887.] 



(Plate XIII.) 



Dr. Anderson collected many specimens of this species from an 

 Hydrophid obtained among the fishing- stakes at Mergui. They 

 were attached to the scales of the snake, and were so numerous 

 as to give a rough appearance to the entire body. On an average 

 the Cirriped measured about 9 millim. in length. 



So far as my knowledge goes, this species has not been observed 

 since Darwin published his description * from specimens obtained 

 in the Indian Ocean, which were also attached to a sea-snake, 

 and for which he was indebted to the kindness of Mr. Busk. 



Although I believe there can be no question as to the identity 

 of Darwin's D. pellucida and the specimens from the Mergui 

 Archipelago, I may be permitted to point out such small differ- 

 ences as I have been able to observe. 



The shape of the valves closely resembles Darwin's description 

 and figures. The basal segment of the scutum is much narrower 

 than the occludent segment, and about half as long. The tergum 

 (fig. 2) has the form of a battle-axe ; its handle, however, is not 

 narrower than the occludent segment of the scuta (as Darwin 

 says it is), but considerably thicker, or about as thick. The 

 occludent segment of the scutum forms with the handle of the 

 battle-axe an angle (fig. 2, a, b, c) of more than 45°. The carina 

 agrees perfectly with Darwin's description, and so does the 

 peduncle, of which Darwin says, "its narrowness and trans- 

 parency are its only remarkable characters." 



The size of the specimens from Mergui considerably exceeds the 

 dimensions given by Darwin. In the specimens he studied the 

 capitulum measured 0'15 of an inch in length, which is about 

 8*8 millim. The Mergui specimen figured (fig. 1) has a capitulum 

 5*8 millim. long, while another measures even 6'2 millim. On 

 the contrary, the peduncle iw only about once and a half as long 

 as the capitulum. Most probably these differences in size, as 

 well as those very inconsiderable ones in the valves which I have 



* Monograph of the Cirripcdiu, joL i. p. 125, pi. ii. fig. 7. 



