14 ON SOME NEW TEST OBJECTS. 



X. 



Ameeican Species of the Genus Monactinus. 



The genus Monactinus differs from Pecliastrum of Meyen [Micrasterias of Ehren- 

 berg), by having but one point or horn, instead of two, to each of the cells com- 

 posing the circumference. Several forms belonging to this genus occur in the 

 sediment filtered from the Croton water in New York, and those here described 

 appear suflQciently constant in character to rank as species. As I cannot satisfac- 

 torily identify them with foreign species, I have ventured to give them names. 



1 . ITIonactinus octouarius, B. 



(Plate, Fig. 36.) 



Circumference composed of eight cells. Centre void. 

 Habitat : Croton water, New York city. 



3. Monactinus dnodenai'iu§, B. 



(Plate, Fig. 37.) 



Circumference with twelve cells. Centre with three cells. 

 Habitat ; Croton Avater, New York. 



XI. 



On some New Test Objects. 



1. Much of the trouble experienced in resolving finely lined objects by oblique 

 light, arises from the necessity for the lines to occupy certain favorable positions 

 with regard to the light, which an unpractised observer may find it difficult to 

 obtain. A test which shall remove this difficulty must be circular, with lines 

 radiating in all directions, so as to require no displacement in order to get the most 

 favorable illumination ; for it is obvious that, on such a body, some portion will 

 always be in the best possible position with regard to the light. A diatomaceous 

 shell, which has lines thus arranged, and which at the same time presents dots and 

 lines of sufficient delicacy, is the Hyalodiscus subtilis, B. (Plate, Fig. 12) from Hali- 

 fax, Nova Scotia. The largest disks are as finely marked as the Greenport Oram- 

 maiopliora, while the smallest specimens are not easier to resolve than the Oram- 

 matopliora suhtilissima, B. from Providence, Rhode Island. It is probable that these 

 Hyalodisci may be found in considerable numbers on our northern coasts, and if so, 

 they will prove admirable test objects. 



2. A test object, which is even more difficult to resolve than either Amici's test 

 or the Providence GrammatopJiora, is presented by what appears to be a variety of 

 Ehrenberg's Orammaiophora siricta from Halifax. Lenses which easily resolved 

 the two tests above mentioned, entirely failed to resolve the Halifax specimens, 

 which only yielded to some new objectives (J^ths) made by Spencer. 



I should here state that, in the spring of 1853, I resolved the Greenport Grain- 

 matophora unmistakably by a ith of an inch objective made by Spencer, and subse- 

 quently by a Hh recently made by Powell, of London, for Dr. Vanarsdale of New 

 York. Mr. Spencer informs me that, since the above observations were made, he 

 has greatly improved his objectives, so that his iths will readily resolve the Green- 

 port Grammafopliora . 



