FOSSIL INFUSORIA. 323 



fossil specimens of Navicula viridis, which consists of four 

 prismatic loricse, two ventral and two lateral ; the former 

 having round, the latter truncate extremities ; and both 

 provided with two rows of transverse markings and dots, 

 longer and more marked on the ventral than on the lateral 

 surfaces. The specimens having their ventral surfaces 

 uppermost, exhibit a longitudinal marking in the centre, 

 with a slight dilatation or knob at each extremity ; this 

 marking is interrupted in the middle of the lorica, and a 

 diamond-shaped spot is left ; if one of the lateral loricse be 

 examined, two of the same spots will be seen, one on each 

 side ; they are of triangular figure, and appear to be thicker 

 parts of the shell, described as holes by Ehrenberg." Four 

 smaller triangular spots may be observed in the same 

 lorica, one being situated at each corner ; these also have 

 been considered as openings by Ehrenberg : their length 

 varies considerably; some exceed the 100th, whilst others 

 are even smaller than the 1000th of an inch. Isthmia ener- 

 ms (fig. 183, No. 5) is usually found attached to sea-weed ; it 

 is in three parts ; and of a trapezoid shape, the centre part 

 appears like a band passing over, and is bounded by broad 

 straight lines : its outer surface is covered with a network 

 of rounded reticulations, arranged in parallel lines. Among 

 the most remarkable are Amphitetras antedilumana ; they 

 are of a cubical or box-like figure, and consist of three 

 portions, the one in the centre being in the form of a band, 

 as shown at fig. 184, No. 8, and the two lateral ones having 

 four slightly projecting angles, with an opening into each. 

 When viewed in detached pieces, the central one is like 

 a box, and the two lateral portions resemble the cover and 

 bottom. The former may be readily known, as consisting 

 merely of a square frame-work with striated sides ; but 

 both the latter are marked with radiating reticulations. 

 When recent, they are found in zigzag chains, from their 

 cohering only by alternate angles. In some instances, as 

 in Biddulphia, and Isthmia, two young specimens may be 

 found within an old one. Cocconeis is marked with eight 

 or ten lines proceeding from the inner margin to the 

 centre ; between which are dotted furrows, with the earlier 

 spot in the centre of each. (Fig. 183, No. 3.) 



Campilodiscus clypeus is oval, and curved in opposite 



Y 2 



