Vol. 51.] OF THE MID-COTTESWOLDS. 457 



(Somerset), and Stoke Knap (Dorset). Then later, in the discitce 

 hemera, appears cortonensis, a feebly uniplicate derivative from a 

 punctata-like form, itself developing biplicate derivatives not unlike 

 the young of the earlier species perovalis. Contemporaneously in 

 the Cotteswolds appears the more-regularly elliptical Buchmani, 

 developing into various biplicate forms, for instance, the associated 

 T. cricJcleyensis ; and then there is the more circular, feebly biplicate 

 Vptoni — a development from a more circular form than Buchmani. 

 Later (Sauzei hemera, Sandford Lane) is the more-inflated-than- 

 Backmani, uniplicate ventricosa, which, according to Deslong- 

 champs's figures (' submaxillata '), appears to develop in many 

 ways (biplicate). Then there is Phillipsiana, of somewhat uncertain 

 exact date — a biplicate derivative from a BucJcmani-like form ; and 

 in the Humphriesiani hemera of Dorset are forms called Buckmani 

 (uniplicate) and Buckmaniana (biplicate) — both somewhat different 

 from the earlier Cotteswold fossils which bear those names. 



Then, though the exact date of PMUipsiana is undetermined, 1 it 

 certainly lived much earlier than Phillipsi. The genesis of that 

 fossil was, first gravida, a tumid, short form, a possible derivative 

 from the punc ta to-stock ; next Cranes, an elongate form ; next 

 Dolliae, feebly biplicate ; next Phillipsi, strongly biplicate. Its 

 contemporary StepTiani (biplicate) came by another and different 

 route as a development of some ventricosa-like form ; and a possible 

 development of Stephani, the T. linguifera of the Fullers' Earth 

 Kock, shows the return to the uniplicate form, with raised fold. 



Here may be noticed the case of T. submaxillata and T. maxillata ; 

 the former is sometimes merged in the same name as the latter, and 

 then T. maxillata is said to range from the base of the Inferior 

 Oolite into the Cornbrash — an absolutely incorrect statement ; 

 because in many hemerae no maxillatoid forms were in existence. 



T. submaxillata occurs in the Oolite Marl (bradfordensis hemera), 

 and unless it be the biplicate development of the uniplicate 

 T. WhitaJceri, its descent is rather uncertain. Then there were no 

 maxillatoid forms for a considerable time — not until the Glypevs- 

 grit is reached (])Gst-Garantianw hemera), when certain biplicate, 

 almost quadriplicate forms make their appearance. Then some time 

 elapsed before the true T. maxillata came. It is in no way a de- 

 scendant of T. submaxillata, or of the maxillatoid forms of the 

 Clypeus-grit, for the former, at any rate, shows greater senility ; 

 but it is, as its own young indicate, a biplicate development of the 

 non-plicate T. marmorea, Oppel ; and it is only in the Cornbrash that 

 maxillata seems to develop senile forms. 



Analogous cases of the heterogenetic development of similar but 

 entirely unrelated forms are well known among ammonites. 

 Mojsisovics has called such cases, between non-contemporaneous 

 ammonites, ' heterochronous convergence.' 2 I would rather apply 



1 See Postscript, p. 461. 



2 'Ceph. d. Hallstatter Kalke,' vol. ii. Abbandl. d. k. k. geol. Eeichsanstalt, 

 vol. vi. pt. ii. (1893) p. 5. 



