CONTAINING DIATOMACEOUS EXUVLE. 41 



truncate extremities of the broad median line. Its habitat is 

 not certainly known, (223.) 



Fiij. 18. NavicuJa crassa, W. 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. Striae transverse, very fine, but distinct, not quite 

 reaching the central line. It is frequent in the deposit, and is 

 probably a marine form. (224.) 



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

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

 linear, elliptical, broad, with obtuse extremities. Striae fine, 

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

 variety which is longer and narrower. Habitat unknown. 

 (225.) 



Fig. 20. Pinnularia iyNavicuIa) Gastnim, 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. Striae 

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

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



Fig. 21. Piiimila.ria apiculata, VV. G. This is another 

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

 Fo'in linear, narrow, contracted to small truncate apices. 

 Striae distant, transverse, hardly reaching the median line. 

 Habitat unknown. (227.) 



Fig 22. Synedi^a Vertebra, W. G. Tliis form, which is 

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

 S. pnlchella 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- 

 tebrae, 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, Si/nedra 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. 



