530 DR. A. W. ROWE ON THE GENUS MICEASTER. [Aug. 1 899, 



held this view, uutil he had examined a large series of hoth types 

 hy the aid of the essential features of the test previously mentioned. 

 The invariable association of the ringed, unjointed peristome, and 

 the scantily tuberculated labral i)late, with the smooth, unsutured 

 interporiferous area, and the equally constant conjunction of the 

 jointed peristome, and the freely tuberculated labral plate, with 

 the ' sutured ' interporiferous area, give the clue to the method of 

 separating the Micr aster LesJcei-type from its transition-form. If 

 the rudimentary tuberculation of the labral plate and the unjointed 

 peristome are mere attributes of immaturity, we should expect to 

 find some evidence of this in its close ally, M. cor-bovls. On the 

 contrary, we find that these two features are as prominent in the 

 largest examples, as in the smallest, of that species. 



Moreover, even in the smallest examples of the passage-form, 

 the tuberculation of the labral plate is always more abundant and 

 symmetrical than in M. LesJcei of the same size ; and in speci- 

 mens of M. j^rcecurso?^ of the Holaster flanus-T^onQ with ' inflated ' 

 plates, even though they be no more than 20 mm. long, the tuber- 

 culation is always linear and symmetrical. It would, therefore, 

 seem that there is ample justification for the separation of M. LesJcei 

 from its passage-form, other than that of mere convenience in 

 classification. 



(4) Grou^ of Micraster pr(BCursor. 

 M. praecursor, 1899.^ 



Dimensions. — The forms of Micraster included in this group 

 are very variable in size, ranging from 26 to 68 mm. in length, and 

 averaging about 50 mm. The breadth is generally about 3 mm. 

 less than the length. Small forms are decidedly rare. 



The writer has ventured to establish a group, rather than to 

 found a species for a single form, as is the custom, for the simple 

 reason that the variations which group themselves around the 

 dominant type are so numerous and so closely linked that any other 

 course would be contrary to fact ; moreover, this plan prevents a 

 needless multiplication of the so-called species. 



^ [It might have seemed more in accordance with ordinary methods to have 

 named this group the Group of Micraster cor-testtidinarium^ that being the older 

 name ; but seeing that the forms answering to M. cor-testicdinarium are com- 

 paratively rare in this country, and that M. prcBCursor is very abundant, I have 

 felt impelled to adopt the latter as the more correct and useful name for the 

 group. 



Further, considering that the narrow forms are found in the zones of 

 Hhynchonella Cuvieri and Terebratidina gracilis, and that no broad forms are 

 Ibund until we get well into the zone of Holaster 'planus, it is clear that 

 zoologically the narrow forms are the older, and that the broad forms are 

 derived from them ; and lastly, the Goldfuss diagnosis is so definite in its stress 

 on the question of breadth that it would be unwise to merge in it forms in, 

 which lack of breadth is an essential feature. — June 7th, 1899.] 



