I90I 



S. S. BUCKMAN— BRACHIOPODA 



233 



earlier one — in which case they are called heterochronous 

 homueomorphs.* 



An analysis of the methods of development pursued 

 by some of the more notable stocks of Jurassic Brachio- 

 poda may be given now, as having a particular bearing on 

 the subject of homoeomorphy. 



II. Methods of Development 



The following tabular arrangement is only a rough 

 analysis ; but it may indicate the methods of develop- 

 ment : — 



1 a. Dorsalt sulcation slight. 



b. Gradual development of 

 gibbous valves. 



c. Later marginal thicken- 

 ing of valves. 



" Tereb ( IV.) Heyseana" Desl. 

 Zeilleria cirat/aris, sp. n. ; Z. 

 Lycetti ; Z. Lecke}ibyi, Z. anglica 



Z. Leckeiibyt ; Z. angltca ; Z. 

 Witchelli. 

 old age forms of above species. 



From stage analogous to \b. 



there is 

 bb. Dorsal plicatioii slight. 



Z. Darwini; Z. disculus. 



A gibbous-valved form analogous to \b, (or perhaps the same, 

 attained by passing through \a, but of this there is no evidence yet,) 

 develops : — 



Zeilleria numismalis ; Z. tnden- 

 tata; Z. digona, etc. 

 Zeilleria cornuta ; Terebratula 

 imitator., sp. n. ; T. eudesiana. 



\b a. Dorsal and ventral sul- 

 cation. 



b. Later in some cases, 

 outgrowth of marginal 

 lobes each side of sulci, 

 produces bilobation 

 (pseudobiplication.) 



c. Later, repetition of bilo- 

 bation produces quadri- 

 lobation. 



• For remarks on this subject, see "The Bajocian of the Mid-Cotteswolds.' 

 Appendix, "Notes on certain Brachiopoda," Quart. Journ. Geoh Soc, Voh li., p. 456. 

 t Used arbitrarily, dorsal = brachial (valve), ventral = pedicle (valve). 



Q2 



Z. quadrifida. 



