232 PROC. COTTESWOLD CLUB vol. xill. (4) 



species nearly alike so far as superficial appearance is 

 concerned, but unlike when particular structural details 

 are closely examined. It is the phenomenon of similarity 

 in general with dissimilarity in details. 



What is known as mimicry in the animal kingdom is, 

 of course, one phase of homoeomorphy. Mimicry may 

 be suggested in regard to homoeomorphous fossil species. 

 I seem to recollect suggesting it some years ago in regard 

 to Ammonites — the genera Ditmortieria and Gramnio- 

 ceras — but I have forgotten where. It would be, perhaps, 

 impossible to prove mimicry in regard to fossils ; but 

 with them homoeomorphy mostly arises from parallelism 

 of development — the tendency of different genetic stocks 

 to pass, quite independently, through similar phases of 

 development — such as the tendency of distinguishable 

 series of smooth Ammonites to become costate, of costate 

 Ammonites to become spinous, in progressive develop- 

 ment ; or in retrogressive development, of spinous Ammo- 

 nites to become costate, of the costate to become smooth. 

 Similarly there is a tendency among Jurassic Brachiopoda 

 for independent non-plicate species to become multiplicate : 

 that is, analogous to the costation in Ammonites; and 

 in the Rhynchonelhdae for the multiplicate (costate) to 

 become spinous (Acanthothyris), and in certain cases a 

 spinous species may, with age, retrogress to lose spines 

 (Acanth. obornensis.)* 



Again, in Jurassic Brachiopoda a simpler development 

 may be found — the tendency of smooth non-plicate species 

 to become uniplicate, of the uniphcate to become biplicate. 



The various species of different stocks may either pro- 

 duce these developmental characters more or less con- 

 temporaneously, in which case such forms are called 

 isochronous homceomorphs, or they may produce the 

 characters at different dates — a later form simulating an 



• "Spinose Rhynchonelhe;' Buckman & Walker; Yorkshire Philos. Soc, 1880, p. 13. 



