Prof. T. H. Huxley on the Development of Pyrosoma. 29 



glass of the compound microscope, presents in the right valve 

 three divergent cardinal teeth, with well-developed laterals on 

 the anterior side, and others more elongated and obsolete pos- 

 teriorly ; in the left valve there are also three cardinals, with, 

 on the anterior side, a strong lateral, and another very incon- 

 spicuous one at the posterior end. The latter (Lepton] in both 

 valves, shows only a single, rather slender, conical, pointed pri- 

 mary tooth projecting subhorizontally inwards, or at an angle 

 of from 60-90 ; and the laterals in the right valve on each side 

 are double, with a single lamina on each side in the left valve. 



Mr. Jeffreys' s scheme of dentition is greatly at variance with 

 the preceding ; but the error of that gentleman may have arisen 

 in consequence of his having only dead, worn, and imperfect 

 specimens, in which the laminar portion of the dentition is often 

 wanting. If a pair of valves of the adult Circe, and another 

 of these young objects, are placed in a parallel position as 

 regards the dentition, it will be seen that in both it is identical. 

 I have found this to be the case in every perfect shell of Mr. 

 Norman's that I have examined. 



The character of the beaks being calculated is a very unim- 

 portant one, as many bivalves in extreme youth are so, but the 

 calix, as the shell grows, wears away ; and even in my live ex- 

 ample, of only 2^ of an inch diameter, the commencement of 

 the sloughing-off is seen ; and it is not till then that the germ 

 of the external ligament is visible, and the true beaks, with 

 their slight anterior arcuation, become established. 



It thus appears that there is not a congruous character in 

 Lepton and Circe ; they differ in contour, sculpture, dentition, 

 ligament in fact, in nearly all points. I therefore think the 

 argument has sufficiently proved that the so-called Lepton sul- 

 catulum is not a new species, and that it is one of the young 

 conditions of the Circe minima. 



I am, Gentlemen, 



Your most obedient Servant, 



WM. CLARK. 



V. On the Development of Pyrosoma. By THOMAS II. HUX- 

 LEY, F.R.S., F.L.S., Professor of Natural History, Govern- 

 ment School of Mines. 



THE following abstract contains an account of some observations 

 which were laid before the Linnsean Society at its meeting of 

 the 1st of December 1859. 



A fine specimen of Pyrosoma giganteum having come into my 

 hands through the kindness of Admiral Fitzroy, of the Marine 

 Department of the Board of Trade, I sought in it for those sin- 



