7928 SYSTEMATIC n 



April 



All the English species are recorded as from the humphriesianum 

 zone : that is to say, that their date is, presumably, about the Epalxttes 

 hemera. The genus is very characteristic of the early Stepheoceratan 

 —Pcecilomorphus macer, S.B., Mon., xxn, 23-29, being excluded from 

 the genus (see Macer atites below). 



The ontogeny of broken-up specimens of the genus reveals the 

 following development. The beginning is a ribless form, with rounded 

 venter and subinflated whorls — very much like Lissoceras oolithicutn 

 (d'Orbigny) in miniature. Later development is to more-inflated whorls, 

 with broadening of periphery, accompanied by ventral change, to 

 angulate, then to carinati-bisulcate, with stronger and stronger furrows. 

 Ribs appear showing well-marked primaries, later these disappear and 

 leave the secondaries which are persistent, though finally fading away 

 to a capillate stage. Meanwhile, the somewhat open umbilicus tends 

 to close up, till it becomes quite small — the next stage would be an 

 opening out again, some signs of which may be detected in P. asper 

 and P. inclusus. From the broad periphery with strong furrows cata- 

 genetic changes are to angulate periphery with loss of furrows — the 

 reverse of the anagenetic development. 



All these changes do not march in step — one feature may accelerate 

 while its companions lag, or vice versa : this makes for variety of form. 

 But all are moving towards a sort of acmic goal, and the acmic form 

 may be considered to be, in strength of ribs, in breadth of venter, in 

 deepness of furrows and in closing of umbilicus, Pcecilomorphus cy chides 

 (d'Orbigny), as restricted and defined above. 



After attaining this acmic goal there is a general catagenesis at 

 unequal rates — decline of rib-strength, narrowing of venter, shallowing 

 of furrows, with slight opening-out of umbilication, or perhaps little 

 change in that character, until such forms as P. angulinus and P. 

 capillaceus are produced. 



It is hoped that it will be possible to return to this subject of the 

 development of Pcecilomorphus, when there is opportunity to prepare the 

 material fully and to take the necessary enlarged photographs of inner 

 whorls of the specimens. 



Maceratites, S. Buckman, 1928, Genotype, M . aurifer, T.A. vii, 

 PL DCCLXVI. At the date of the Monograph, 1889, see Pcecilomorphus 

 (p. 9, above), I supposed that the forms which are now included in this 

 genus might be ancestors of the later species of Pcecilomorphus through 

 P. evolutus (p. 10, above) ; but this idea cannot be sustained now. 

 In the macer group (Maceratites) the proportions are different, the ribbing 

 does not correspond — the ribs being rarely furcate and quite pronounced 

 on the inner area — and the mouth-border does not agree : in Maceratites 

 there is a pronounced spatulate lateral auricle, in Pcecilomorphus nothing 

 but a plain curved mouth-border has yet been seen, the rib-curve giving 

 no indication of any extrusion, in the middle lateral area, indicative 

 of lateral auricles. 



There are now figured three different forms at least : — 



1. Maceratites aurifer, T. A., PI. DCCLXVI, Genotype and Holo- 

 type, agreeing with Pcecilomorphus macer, paratype (Mon. xxn, 27, 28), 

 somewhat stout and rather coarsely ribbed. 



2. Maceratites costulatus, nov. = P. macer, paratype (Mon. 

 xxn, 25, 26, Holotype of M. costulatus), approximate ribbing with 

 obovoid aperture. 



3. Maceratites macer, (Mon. xxn, 23, 24), Holotype of P. macer, 

 S.B., 1889; thin, smooth-looking, the ribs being of low relief. 



