of the Bivalve Mollusca. 245 



while another flows off posteriorly near the termination of the 

 branchise*. 



I now come to the most interesting part of Mr. Clark's letter, 

 where he informs us that he has ascertained that Kellia subor- 

 bicularis is viviparous, — a good discovery : but the supposition 

 that the anterior siphon is only intended as a marsupial pouch 

 for the further development of the ova after their extrusion from 

 the ovarium, is a conjecture not warranted by Mr. Clark's own 

 observations, as he afterwards saw completely testaceous young 

 in the ovarium, thus doing away with the necessity of their 

 being further detained in the open siphon, which is ill-adapted 

 to the office assigned to it. Besides, if such had been the case, 

 it would most likely have been observed before, as from the 

 hyaline transparency of the tube and its wide aperture, it is 

 always easy to see to the bottom of it. That the young escape 

 by this aperture is probable, but this does not prevent its being 

 used for branchial purposes ; as in no instance that I am aware 

 of, either in a Bivalve or an Ascidian, is there a separate orifice 

 of the cloak set apart for the extrusion of the ova. All that can 

 therefore be admitted as proved by Mr. Clark's observations, are 

 the viviparous character of the reproduction in Kellia suborbi- 

 cularis and the escape of the young (in one instance at least) by 

 the anterior siphon. May I not add, — it is also proved by equally 

 authentic observations, often repeated, — that both in Kellia rubra 

 and K. sub orbicularis, a special current can be seen to go into 

 this siphon, and at no other part of the circumference of. the 

 mantle ? 



I remain, dear Sir, yours very truly, 



Joshua Alder 



P.S. Since writing the above I have had an opportunity of 

 examining the currents in Pholas crispata, which I find to cor- 

 respond entirely with those of the species already mentioned. 

 As however Mr. Garner, in his excellent paper on the Lamelli- 

 branchiata, though agreeing in the general existence of ciliary 

 currents received and expelled by separate apertures, yet consi- 

 ders this and some other allied genera to be exceptions, I pur- 

 pose, with your permission, to treat this part of the subject a 

 little more at large in a separate communication. 



* With respect to the range of Kellia rubra, Mr. Clark has ascertained 

 that, he was right in stating that near Exmouth this species is found beyond 

 ordinary high- water-mark, and often, in calm weather, is only covered by 

 the sea at spring tides. If it has been also ascertained that " thousands of 

 these animals pass their entire existence without perhaps being completely 

 in a condition to receive branchial currents of sea-water," I shall agree 

 that I was mistaken in thinking the account in question overstated. The 

 ordinary range of Kellia rubra is within tide-marks. 



