14 POST-TERTIARY LACUSTRINE SAND 



for a long way, of uniform width, and terminates in small 

 oval expansions. The narrow part has strong moniliform 

 stria?, which, in the central and terminal expansions, are 

 resolved, except just at the margin, into a general granulation. 

 The margin is undulated, except for a short distance from 

 each apex. It will be seen by one of the figures, which is 

 not so long as some are, that the narrow part, on one side, 

 without any part of the central long expansion, is frequently 

 so long as to extend the whole way across the field, with a 

 power of 400, that is, probably, l-50th to l-40th of an inch. 

 This would make the length of the entire form to be probably 

 from the l-20th to the l-15th of an inch, or more. This, with 

 its extreme tenuity, accounts for its not occurring entire in a 

 deposit carried by water, where it must have been constantly 

 agitated. I have been informed by Mr. West, that a similar 

 form, possibly of the same species, although shorter, occurs in 

 a gathering from Port Natal, in the hands of Mr. Shadbolt. 

 This curious Synedra is, therefore, a marine form, and I 

 anticipate that it will be found recent on our own coasts. 

 (229.) 



Having now briefly described the new forms in the Glen- 

 shira sand, so far as they are here figured, I am compelled to 

 postpone the remainder to the next number of the Journal, 

 in which another plate will be required for them, as very 

 nearly as many remain to be described as we have now been 

 enabled to figure. In the meantime, besides the Eiuwtia 

 Falx, which is not yet a British form, we have described 18 

 new forms, all from this one deposit, and one new to Britain. 

 These, added to the list of known forms, make up the 

 number of 229 species now recorded as occurring in the 

 Glenshira sand, besides those to be hereafter noticed and 

 figured. 



It may be noticed here, that I intend to publish, as soon as 

 the necessary figures can be prepared, a description of a very 

 remarkable series of forms, occurring both in the Glenshira 

 sand, and in various fresh- water gatherings, in which, indeed, I 

 first observed them. They agree perfectly in general aspect, 

 and the peculiar characters of the markings ; but differ to a 

 very surprising degree in form or outline. These may possibly 

 constitute several species, and would certainly be considered 

 as such by some authorities. But, both on account of their 

 resemblance, or rather identity, in markings, and from the 

 occurrence of intermediate or transition forms, by which the 

 different types appear, in many cases at least, to pass into 

 one another, there is some ground for regarding them as be- 

 longing to one species. Without deciding this question, 1 





