﻿46G DR. A. VArGHAN OX THE FAUXAL SUCCESSION [Aug. I908, 



II. EliACHIOPODS. 



[rsoTE. — ' Dayidson ' sig-.iifies DaTidson, ' Monogr. Brit. Foss. Bracb.' vol. ii 

 (1858-63) Palseont. Soc] 



Peodtjctids. 



Productus. 

 PRODTJCxrs PEOBosciDEUs, de Terneuil. (PI. L, fig. 1.) 



Davidson, pi. xxxiii. figs. 1-4, & Suppl. vol. iv (1874-82) pi. xxxvi, fig. 13. 



This species occurs only near the top of the Dihunophyllum-ZonQ. 

 Davidson records it from Settle and Drogheda. Dr. Wheelton 

 Hind has collected a few specimens from Park Hill and Castleton. 

 The specimen here figured was collected by Dr. Matley from 

 Lower P (L 5 d), where the species is apparently not uncommon. 



The general type of ribbing and wrinkling immediately recalls 

 Productus undatus^ and the following facts throw light on the line 

 of evolution which has resulted in the remarkable form under 

 discussion : — 



(1) In Sj and Lower S., of the South-Western Province, a trans- 

 versely-wrinkled variant from the stock of Productus cormigato- 

 liemisphericus is common (see Proc. Bristol Xat. Soc. vol. x, 1903, 

 pp. 131-34 & pi. ii, fig. 4). 



(2) Mr. J. T. Stobbs has sent me a large number of specimens of 

 typical Productus undatus from Eedesdale (Xorthumberland), where 

 the species occurs abundantly ; many of these approach very 

 close to the specimens from the South-AVestern Province mentioned 

 above. 



The associated corals indicate D^.^ 



(3) Specimens of Productus undatus from immediately below the 

 Hardraw Limestone of the Yoredale Province (D^), sent to me by 

 Mr. Cosmo Johns, exhibit great variability and elongation of form. 



Piecing together these several facts, the history of Productus 

 prohoscideus is probably much as follows : — The ancestral stock of 

 Pr. cor7'U(/ato-hemisphericus emitted in early S-time a continuously- 

 wrinkled variant, the characters of which became more and more 

 firmly impressed until, in late S and early D-time, Pr. undatus 

 reached its maximum stability and abundance. In late D-time 

 the gens became moribund, and its characters therefore unstable, 

 resulting in rapid variation and great irregularity. 



Peoductus steiatus (Fischer). (PL L, fig. 2.) 

 Compare Davidson, pi. xxxiv, figs. 1 <t 3. 

 This species is abundant in Lower and Middle P of the Rush- 



1 Prof. Barrois informs me that Productus undatus characterizes beds in the 

 Belgian sequence which are probably referable to S or D^. 



