.524 DK. A. W. KOWE ON THE GENT7S MIORASTER. [Aug. 1 899, 



direct hito Micraster Leslcei, and into it alone, and that it h^-s iio 

 direct zoological association with the other low-zonal forms : these 

 latter, in turn, being directly derived from M. LesJcei. 



Distribution. — At Dover this Micraster has been found 

 throughout the upper 90 feet of the Terebratidina gracilis-zone, and 

 it probabl}^ extends lower still. It occurs from top to bottom of the 

 Holaster jjlanus-zowQ^ and then suddenly dies out. A solitary 

 example has been discovered 30 feet up in the M. cor-testwUnarium-- 

 zone. It has not been found in the Bhynchonella Cuuieri-zone, in 

 the West Cliifs, but the search has not been so exhaustive there as 

 an the East Cliffs. 



At Beachy Head it has been found only in the T. gracilis and 

 H.'planus-zoneB, and then only in small numbers. At Beer Head, in 

 South Devon, it is seen throughout the entire extent of the T, gracilis 

 ^nd H. planus-zoneB, and it is quite as abundant as at Dover. 

 Examples have been obtained at the base of the T. gracilis-zone, a few 

 feet above the junction with the beds of Rh. Ouvieri. On the Dorset 

 coast it was found near the top of the gracilis-heds, and in the 

 zone of H. planus. Fragments of a small thin-tested urchin have 

 been found in the Rli. Ouvieri-zone at Beer, but sufficient material 

 has not yet been obtained to allow of a reference of these fragments 

 to the genus Micraster. 



A full description of the diagnostic features of this interesting 

 Micraster has been given, because the writer has been unable to find 

 any account of it which does justice to its strong individuality. No 

 Micraster, save M. Sanctoi-MaurcB, Gauthier, is so sharply marked 

 out from its fellows, and has so strong a claim to be stamped as a 

 distinct and definite species. Even the minor points which have been 

 brought out are worthy of notice, and no single one is immaterial 

 to the formation of a determination. It will be noticed that stress 

 has been laid on the importance of the thinness of the test, and it 

 will be apparent that many of the salient features, upon which we 

 rely for a determination, are also due entirely to this cause. 



M. cor-bovis has all the low-zonal features, which are so strongly 

 exemplified in M. prcecursor and M. cor-testudinarium, and that in 

 an exaggerated degree ; and, in addition, many of the fixed features 

 of the test are rudimentary to a notable extent. It will, therefore, 

 be not unreasonable to look upon this primitive form as the pro- 

 genitor of M. Leshei, and through it of M. prcecursor and M. cor- 

 testudinarium. 



Owing to the necessarily limited space, and to the excellence of 

 the plates of the large forms by Forbes and Wright, it is here 

 sufficient to give the profile figures on PI. XXXV. Photomicro- 

 graphs of the essential characters of the base, upon which one must 

 rely for a determination, will be found, however, in PI. XXXYIII, 

 fig. 5, and PI. XXXIX, figs. 1-4. 



