Yol. 55.] DK. A. W. KOWE OX THE GENUS MTCRASTEK. 535 



Bayle ; and that in the true Goldfussian type the pores of the single 

 ambulacrum are unequal, while they are rounded in Micraster 

 deciijiens. In this paper it has been decided, however, to retain 

 the name of M. cor-testucUnarium for the broad low-zonal varieties 

 of the English Chalk. 



Tbis broad variety shares the general features common to it and 

 all the low-zonal forms, a summary of which will be found on p. 516. 

 Just as M. prcecursor has special features, according to the zone in 

 which it occurs, so does M. cor-testudinarium vary in identical 

 degree. Save only breadth and size, it has no general or special 

 features that set it apart from the narrow-shaped M. prcecursor- 

 group. It is merely a broad and large low-zonal form. When it 

 is noted that all broad forms, whether they be high-zonal or low- 

 zonal, have a tendency to assume a large size, it will be clear how 

 little claim mere breadth and size have in constituting a specific 

 distinction. 



Special features of the test. — There are no features 

 special to this variety, but only features which are distinctive of 

 the horizon at which it exists. In this matter it is on all fours 

 with M. pr(ECursor, and, to avoid repetition, the reader is referred 

 to the description of the features of that group on p. 533. It is 

 just as easy to refer J/, cor-testudinarium to its zone as it is in the 

 case of the narrow forms, for its zonal distinctions are identical. 

 These facts alone should suffice to establish it as merely a broad 

 variety of M. prcecursor. 



Variation in shape. — The profiles are the same as we find in 

 all low-zonal forms, and one must again refer to PI. XXXV, lines iii 

 &> V, where it will be seen that the broad forms in each zone are, 

 in their profiles, merely the echo of the narrow forms in the same 

 zone. As far as mere shape is concerned, it will be seen, on line 

 vii, that even in the high-zonal series much the same general out- 

 line can be traced, with the addition of certain features peculiar to 

 the horizon. Examples coinciding with J/. Gosseleii^ Cayeux, 

 M. decijnens, Bayle, and M. turonensis^ Eayle, should, in the writer's 

 opinion, be grouped under the heading of M. cor-testudinarium, as 

 they appear to be merely prominent variations and not specific 

 types. The first is from the M. cor-testudinarium-zonQ, but the 

 last two have no zone given in Bayle's plate. Any one of them 

 could be referred with equal truth to either the zones of Holaster 

 planus or M. cor-testudinarium, and the last two {M. decipiens and 

 M. turonensis) certainly extend as far up as the base of the M. cor- 

 anguinum-zone. It may be mentioned that aJl the gibbous forms 

 figured in PI. XXXV have well-developed fasciolcs. Just as in 

 the narrow M. prcecwrsor-group, we have in the broad forms a 

 parallel series of profile-shapes — the very depressed forma i\^or- 

 rnannice, the flat-arched forma planidorsata (the true Groldfussian 

 type), the forma heonensis, the forma carinata, and the forma 

 gihhosa. There is one gibbous form, however, needing special 

 mention, which is figured by Wright in 'Brit. Cret. Echinod.' 

 Monogr. Pal. Soc. pp. 335-338 & pi. Ixxvi, fig. 1 a-c. ^This is the tall 



