clxxxiv INFERIOR OOLITE AMMONITES. 



Description. — Subplatyleptogyral ; latumbilicate ; spissiparvicostate ; periphery 

 subacutifastigate, parcicarinate. 



Distinction. — From Dum. signata, the larger umbilicus, as mentioned in p. 252. 

 The costse are also rather more distant and distinct. 



31. Dumortieria exigua, S. Buchnan. Plate XLIII, figs. 11, 12. Suppl., Fig. 184, 



p. cxcvi. 



1891. Dumortieria radians, var. exigua, This Monogr., PL xliii, figs. 11, 12. 



Description. — Subplaty-subleptogyral ; latumbilicate ; spissiparvicostate to 

 striate ; periphery fastigate, par vicar mate. 



32. Dumortieria sp. Plate XLIII, figs. 1 — 4. 



1891. Dumortieria radians, This Monogr., PI. xliii, figs. 1 — 4. 



Remarks. — A young example depicted to show the ontogeny. It has a larger 

 umbilicus than the last species. 



SuBFLEXIRADIATE. 



In this group the radial line has a slight curve on the lateral area ; the umbili- 

 cus is large, and considerable compression is attained even while a strongly costate 

 stage is present. These are all factors of distinction from true Dumortieria, where 

 the radial line is straight, at least until the striate stage is well advanced, where the 

 umbilicus tends to contract as soon as compression commences, and where such com- 

 pression does not become pronounced until costal degeneration has nearly attained 

 the striate stage. 



The comparative development may be expressed somewhat in this way : In 

 the flexicostate series the prisca style of distant ribbing and wide umbilication are 

 found persisting until considerable whorl compression has been reached. So in the 

 recticostate series the prisca style of ribbing is associated with circular whorls, in 

 the flexicostate series with compressed elliptical whorls. 



The present group admits of division into two series, but only in regard to 

 size : 



(1) Latescens series, large forms. Dum. latescens, Dum. arenaria, which is 

 perhaps young of a larger form, and the species called Dum. subundulata, var. 

 striatulocostata by Haug in ' Polyniorphidas,' N. Jahrbuch f. Mineral., 1887, Bd. II, 

 PI. V, fig. 4. 



(2) Subundulata series, all seem to be dwarf forms, Dum. subundulata and its 

 allies. 



