180 HKXBRY, ON HYALODISCUS SUBTILIS. 



Dr. Bailey figures Hyalodiscvs subtilis witli tliroe sets of 

 striation, id est, one set radiating, and two others of contrary 

 curvilinear description, the three sets collectively and their 

 several intersections constituting a most rare, elaborate, and 

 beauteous design. 



Neither Bailey nor Pritchard represent any striation over 

 the central umbilicus, but figure this portion as being merely 

 coarsely granular, whereas I would direct more particular 

 attention to the varied appearance of this central part under 

 different foci and illumination, as an index to the entire 

 phenomena throughout the disc, for just as striation is seen 

 upon the umbilicus, w^hether radiating or curvilinear, so does 

 it prove, on closer examination, indicative of the course or 

 direction of striation found upon the outer bordei', and it is 

 in a measure the varied intersections upon this part, which at 

 times yields a comparative coarse and confused granular 

 aspect, although the central and exterior markings will be 

 ultimately found to be continuous, or prolongations one of 

 the other. 



The aranular condition of the umbilicus contrasted with 

 its superficial striation^, would in some mstances, seem to con- 

 stitute a substratum ; a granular condition being also fre- 

 quently found to pervade the entire shell, as if arising from 

 some abnormal development, or as having been subjected to 

 some accidental or extraneous agency ; and it is much this 

 condition of things so frequently existing amidst these pro- 

 ductions from certain localities, which detracts so greatly 

 from the beauty of structure, and lessens interest attached to 

 this peculiar diatom, or otherwise there exists other species 

 not exhibiting the curvilinear markings of Professor Bailey. — 

 A matter of no little importance suggests itself; do the 

 figures represented by Bailey and Pritchard refer to the 

 entire group of lines mtli their several intersections as 

 capable of being seen under the objective simultaneously, at 

 any one given portion of the shell, or merely as being in part 

 and variously developed on several different portions, thus 

 rendering it necessary, ideally to build iip, by collecting or 

 associating such diversified representations to constitute a 

 whole ? For my own part, I have not hitherto been enabled 

 to resolve the threefold delineation completing tlie figure at 

 any single point at one and the same moment, that is to say, 

 the two diverging (decussating) and the radiating lines one 

 and all simultaneous!}' intersecting ; Init have always occasion 

 to resort to the expedient of revolving the slide so as to pre- 

 sent the object under different aspects relative to illumination, 

 whereupon alone I have succeeded in obtaining a striation 



