ANIMAL NATURE OF DIATOME.E. 477 



states (lorica perforata). The observation that, by placing 

 the object alternately nearer to, or more distant from, the 

 microscope, the cells and their intermediate spaces appear 

 successively in light and shade, is sufficient to prove that 

 the appearance is produced either by simple depression or 

 elevation, causing a difference in the surface, or perhaps 

 by minute cavities in the thickness of the wall itself. 



Kiitzing takes no notice of the marginal perforations 

 described by Ehrenberg, who enumerates as many as 

 twenty-five in C. lineatus. 



Ehrenberg describes the internal substance, either 

 collected into a globular mass in the centre, or radiating 

 irregularly all round from the centre towards the circum- 

 ference, or more frequently arranged in small distinct 

 globules, like those of Melosira. He sees, also ( C. patina), 

 a globular gland, and a contractile diaphanous area which 

 he believes to belong to the male sex. 



59. ACTINOCYCLUS. Individua solitaria, liber a; lorica 

 bivalvis silicea, disciformis (breviter cylindrical cettulosa; 

 cellulce radiis pluribus Itevibm interrupts. Septa interna 



In a specimen of chalk marl, from Oran, in my pos- 

 session, which consists almost entirely of Coscinodisci, 

 Dictyocliae, Melosirte, and spicula3 of sponges, I could 

 not possibly find a single Actinocyclus, although seven 

 out of twenty-four species of this genus are indicated in 

 this substance. This proves only one circumstance, 

 which, for other reasons, can easily be imagined, that in 

 the same formations the species and genera appear to be 

 different at different points. I think, also, that I ought 

 to introduce two observations equally applicable to this 

 genus and both the preceding and subsequent genera. 



Among the innumerable valves of most species of 

 Coscinodiscus, more or less regularly curved, many are 

 found to be flexuose or curved in various ways ; there is 

 this variety of curvature in valves perfectly similar to 

 each other in every other character. Considering, then, 



