Yol. 55.] DR. A. W. EOWE 0:!T THE GEJ^TJS MICRASTER. 519 



important a species, the description is singularly lacking in point. 

 The imperfect description is probably the cause of the species being 

 so little understood, as the writer has met with no English palaeon- 

 tologist who could with certainty distinguish a small Micraster 

 cor-bovis from a large M. Lf-slcei^ and yet they are as easy to 

 separate as M . prcecursor and M. cor-angninum. 



This form is not common in any of the English sections men- 

 tioned in the opening sentence of this paper, and its range is from 

 the base of the Terehratulina f/racilis-zone to the top of the Holaster 

 plamis-zone : one example only has been found in the middle of 

 the M. cor-testudinarium-zone. It is commonest in the gracilis- 

 beds, where, save for a few M. LesJcei at the top of the zone, it is the 

 only Micraster found, and it ranges through the H. jjlamis-zone, 

 from top to bottom, including the Chalk-rock. The large forms are 

 generally obtained at the top of the zone of H. planus, where one 

 was obtained measuring 81 mm. in length ; but Gen. C. E. Cockburn 

 has found the large form well down in the gracilis-heds at Dover, 

 where, as a rule, they rarely exceed 50 mm. in length. 



It has been suggested that the large and small forms are two 

 distinct varieties — the small form being found chiefly in the T. gra- 

 cilis-zone, and the large at the top of the R. planus-zonQ ; but 

 Gen. Cockburn's large example from the T. graciUs-beds, and the 

 fact that the small forms from the H. planus-zone in no way 

 differ from those in the zone below, negative this idea. The large 

 form is looked upon as a mature stage of the small form, reaching its 

 maximum development at the top of the II. planus-zone, and then 

 suddenly dying out. On the whole, bulk for bulk, the small forms 

 are taller than the large : a by no means remarkable fact, when 

 one considers that young forms are generally tumid. 



As Forbes's description is so meagre, it has been thought 

 necessary to give here the results of a detailed examination of a 

 number of zonally-collected examples. 



Test. — This is very thin, thus accounting for the rarity of 

 perfect examples. Even the large specimen, 81 mm. long, figured 

 on the extreme right of line ii, PI. XXXY, has a test as thin as 

 that of H. planus, and this is, in the writer's experience, an un- 

 var3-ing feature, and one of high specific importance. Though M. cor- 

 hovis is a comparatively rare fossil, it is commoner than is generally 

 supposed, as many broken examples are mistaken for specimens of 

 H. planus when seen in situ, and nothing short of extractinij; them, 

 and cleaning a portion of the test in the field, will suffice to determine 

 the difference. No other Micraster^ however small, has a test so 

 thin, and this feature alone is sufficient to establish a determination. 

 This notable character escaped the attention of Forbes, and of other 

 keen observers. 



Size. — M. cor-bovis attains a larger size than any other Micraster 

 in the English Chalk ; but large forms are comparatively rare. 

 They may be readily mistaken for examples of Holaster placenta, Ag., 

 when seen in section in the cliff. 



