Vol, 55.] DK. A. W. EOWE ON THE GENUS MICRASTER. 543 



This diagram needs a word of explanation, as it is impossible to 

 put in' an alternative line of descent without running the risk of 

 confusion. It merely lays claim to be a diagrammatic statement 

 of what obtains in nearly every English section. 



All personal work in the field, and all the information collected 

 from other English sources, clearly point to M. cor-hovis as the only 

 Micraster found below the level of the zone of Holaster planus. The 

 sole exceptions are the rare occurrence of M. LesTcei in the zone 

 of Rhynchonella Cuvieri at Beer in South Devon, and one solitary 

 example found 20 feet down in the Terebratulina gracilis-zojie at 

 Dover. It is a curious fact that not a single fragment of 31. LesJcei 

 was found in the T. gracilis-zone at Beer, though minute search 

 was made for it, while 31. cor-hovis was present in considerable 

 abundance. This gap in the continuity of a species is remarkable, 

 for it comes in with a rush as soon as we enter the U. planus- 

 zone. Through the courtesy of Mr. C. J". A. Meyer, the writer 

 has been able to examine several Micrasters from the Beer district, 

 and he has no hesitation in referring them to 31. Leskei, and not 

 to 31. cor-hovis., as had been done by Mr. Meyer. Therefore, we 

 may look upon the appearance of M. cor-hovis in the T. gracilis- 

 beds, and of 31. LesJcei in the zone of II. planus, as the normal 

 condition in England, so that the diagram is substantially accurate in 

 this particular. The other lines of descent are purely an expression 

 of the personal opinion of the writer, and the reasons for such con- 

 clusions will be found in the text. The unique appearance of 

 31. LesTcei in the Rh. Cuvieri-zone at Beer Head finds its parallel in 

 the occurrence of the same species above the beds of Cidaris hiruclo, 

 in the Yonne, where, according to M. Lambert, it is very rare. 

 According to the same authority, 31. LesJcei is much commoner in 

 the zone of T. gracilis in France than it is in England. The writer 

 has been unable to consult M. Lambert's monograph on the genus 

 3Iicrasfer, as he failed to obtain a copy, and he is the more grateful 

 to the author for information contained in letters. 



Until within the last few weeks the writer believed that he was 

 alone in his views as to the evolution of this genus, but a little 

 tract by Mr. Meyer, on Micrasters in the English Chalk (Geol. Mag. 

 1878, pp. 115-117), sent to him by the author, makes it clear that 

 Mr. Mcvcr had already conceived similar ideas, though they were 

 never elaborated. Curiously enough, it was the Dover section, with 

 its vast zonal range and gentle dip of beds, coupled with its profusion 

 of Micrasters, which inspired us both with the same suggestive facts. 



A word of excuse may be offered for the illustratious of this 

 paper, as they are the productions of an amateur in an almost un- 

 trodden field. The photomicrography of opaque objects is in its 

 infancy, the difficulties of technique are almost insuperable in some 

 instances, and the whole of the apparatus for taking these plates 

 has had to be specially devised. There are limitations to the powers 

 of a, ieiis-, and it is impossible to make it sharply record detail which 

 lies Qii, widely separated horizontal planes — such as the labral plate 



