360 Dr. G. C. Wallich on the Structure of 



been strained a little too far in the attempt to deduce, from the 

 appearances presented by one group of Diatoms which happens 

 to furnish structure easily resolvable even with the lowest powers 

 of the microscope, and the least perfect optical combinations, the 

 nature of the markings in another group which happens to pre- 

 sent a structure that is resolvable with the utmost difficulty. 

 It is with no little satisfaction, therefore, that I find Dr. Carpen- 

 ter, in the recent edition of his work on the microscope (1863), has 

 relinquished the view formerly supported by him regarding the 

 analogy subsisting between such forms as Triceratium, Isthmia 

 and Pleurosigma, and that this eminent authority has given in 

 his adhesion to the opinion long advocated by me, namely, that 

 the valvular surface of P. angulatum is not studded with depres- 

 sions, but with elevations. Moreover it is a significant fact that 

 the woodcut representing the enlarged photographic view of this 

 Diatom inserted in previous editions of his work is altogether 

 omitted in the last one. Of this Prof. Schultze was obviously 

 ignorant when he penned his remarks. Dr. Carpenter does not 

 state, however, whether he considers these elevations as being 

 simply tubercular or formed with distinct sides, that is, with facets, 

 as suggested by me. Since writing my former paper, so far from 

 meeting with any appearances or facts to cause me to modify 

 the view then offered with regard to P. angulatum, that view has 

 derived the strongest confirmation from repeated and careful re- 

 examination of the structure under every condition capable of 

 throwing light upon its true nature. But, owing to the extreme 

 minuteness of the markings of this Dintom, it seemed almost 

 hopeless to render them amenable to anything approaching proof 

 until some closely allied variety should turn up, of sufficient size to 

 meet the requirements of the case. Fortunately some specimens 

 oi P . forma sum^ supplied the necessary conditions. One valve 

 in my possession measures in length joth of an inch, the dia- 

 gonal striae being 25 in '001. Numerous other specimens mea- 

 sure as much as jjth and ^j^yth of an inch, their striation being 

 only a slight degree finer. By means of these, I have been 

 enabled with ease to see the various series of lines, under a 

 Rosses ^-inch objective. With the same eminent maker's lenses 

 of jth, ^th, ^th, and ^th focus, and with a Hartnack's No. 10 

 immersion lens, whether employing direct or oblique, natural or 

 artificial light, and with either shallow or deep eye-pieces, not 

 only is the lozenge-shaped character of the interspaces unmis- 

 takeably determined, but the angulated structure of the elevations 



* Dredged by me at Guernsey in about 15 fathoms of water. According 

 to the characters given in the Synopsis, this Diatom is intermediate be- 

 tween P.formosum and P. decorum. The distinction drawn between these 

 varieties, however, is too trivial to be tenable. 



