CONTAINING DIATOMACEOUS EXUVI/E. 13 



truncate extremities of the broad median line. Its habitat is 



not certainly known. (223.) 



Fig. 18. Navicula crassa, YV. G. This is a fine and well- 

 marked species. Form elliptical, with a very slight inflexion 

 before the obtuse apices. It varies considerably in size ; has 

 a very stout habit, and a brown colour in balsam. There is 

 a large round spot in the centre, within which the two halves 

 of the median line terminate in small round knobs, but do not 

 meet. Stria? transverse, slightly inclined, very fine, but dis- 

 tinct, not quite reaching the central line. It is frequent in the 

 deposit, and is probably a marine form. (224.) 



Fig. 19. Navicula maxima, W. G. This is a fine large 

 form, much less frequent than any of the preceding. Form 

 linear, elliptical, broad, with obtuse extremities. Stria? fine, 

 transverse, reaching the central line. There seems to be a 

 variety which is longer and narrower. Habitat unknown. 

 (225.) 



Fig. 20. Pinnularia {Navicula) Gastrum, Ehr. This little 

 form is new to Britain, having been found by Ehrenberg in 

 Mexican and North American gatherings. It is short, broadly 

 lanceolate, with obtuse extremities slightly constricted. Stria? 

 distinct, strongly radiate. The habitat is not given in Kiitz- 

 ging, but it is probably marine. (226.) 



Fig. 21. Pinnularia apiculata, W. G. This is another 

 well-marked little species, which is not rare in the deposit. 

 Form linear, narrow, contracted to small truncate apices. 

 Stria? distant, transverse, hardly reaching the median line. 

 Habitat unknown. (227.) 



Fig 22. Synedra Vertebra, W. G. This form, which is 

 very frequent in the deposit, belongs to the same division as 

 S. pulchella and S- acicularis. It differs, however, from both 

 these forms, which also occur in the deposit, and can thus 

 be compared with it, in the remarkable relative width of the 

 central portion, which has a somewhat curved outline, and 

 the equally remarkable way in which it suddenly contracts to 

 the very slender terminal portions. In the largest specimens, 

 these are very long. Its form resembles that of certain ver- 

 tebra?, and it has been named so as to recal this resemblance. 

 Nodule strongly developed. Striae very fine. The habitat of 

 this species is unknown. (228.) 



Fig. 23. Synedra undulans, W. G. This is, perhaps, the 

 most remarkable of all the forms in the Glenshira sand. It 

 is exceedingly elongated, and so slender that a perfect 

 specimen has not yet occurred to me. It consists of a middle 

 portion rather wider than the rest, tapering both ways to a 

 very small width. From this point it extends on both sides, 



