ANIMAL NATURE OF DIATOME.E. 483 



cellular siliceous external membrane, which is lacerated in 

 the deduplication. A portion of it may remain in the 

 middle, whilst its lateral extremities are denuded, or vice 

 versa, the former may remain denuded, and the latter 

 covered. It still remains to be explained how the 

 formation of the new valves takes place, whether they are 

 organised in situ, or project by degrees from the companion 

 valves, to which they are contiguous from the beginning, 

 the intermediate ring only developing itself subse- 

 quently (consequentemente), as in Melosira. 



The gelatinous isthmus, which connects the individuals 

 together, and the stipes by which the entire chain 

 is affixed, seem to prove the presence of an exterior 

 gelatinous membrane. 



66. ODONTELLA. Individua laevia, compresso tere- 

 tiuscula medio fasciata utroque apice cornibus binis late- 

 ralibus instructa et concatenate, adnata. 



I am compelled, by the want of specimens, to confine 

 my examination to that one only of the five species of 

 this genus which I have already treated upon in refer- 

 ence to my Pleurosira thermalis ; and I can only insist 

 upon what I have stated on that occasion. The indi- 

 viduals of Odon tella polymorpha, have an elliptic cavity 

 in the middle, and at the extremities are compressed in 

 the direction of the greater diameter of that ellipse ; but 

 viewed laterally, they present an obtuse, but linear edge 

 (canto). The lateral processes, by which the individuals 

 are connected together, are very evident. The difference, 

 therefore, between this form and that of Pleurosira 

 thermalis, and of P. Baileyi, is very great. Another 

 important difference exists in the conjunction of the 

 median ring with the two lateral valves. In my Pleu- 

 rosira there is on both sides a large, distinct, circular 

 canal (canaletto], evidently projecting into the inner 

 cavity, and corresponding to an external furrow. But in 

 the Odontellce there exists neither furrow nor projection ; 

 and I reallv doubt whether there be a fine canal. A 



