PRINCIPLES OF MORPHOGENESIS 17 



man" applied the principle of parallel development for many years and chiefly among the Jurassic 

 ammonites and brachiopods; and his work and now ours show that external characters are not always 

 reliable for generic and phyletic studies. The application of this principle to the genera heretofore 

 described often plays havoc not only with them, but above all with our notions of their genetic 

 relationships. 



This phenomenon of parallel development Buckman has called homceomorphy. It refers to 



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, [or] the tendency of 

 different genetic stocks to pass, quite independently, through similar phases of development . . . There is a ten- 

 dency among Jurassic Brachiopoda for independent non-plicate species to become multiplicate . . . and in the 

 Rhynchonellida: for the multiplicate (costate) to become spinous {A catithothyris) , and in certain cases a spinous 

 species may, with age, retrogress to lose spines. 



The various species of different stocks may cither produce these developmental characters more or less con- 

 temporaneously, in which case such forms are called isochronous homaeomorfhs, or they may produce the char- 

 acters at different dates — a later form simulating an earlier one — in which case they are called heterochronous 

 homoeomorfhs (1901, pp. 231-233). 



In the Jurassic of the Cotteswold district of England, in rocks of about the same date, there are 

 five independent developments of the same character among species of Terebratulidas. In nearly 

 related stocks much excuse may be made for errors in identification, but when such errors occur in 

 different families whose internal structural details are quite distinct, then "the confusion of two species 

 of these families under one name becomes serious" (p. 239). In fact, among the Jurassic terebra- 

 tulids and rhynchonellids, homoeomorphs are so common that "they may be said to form veritable 

 'traps' in the matter of identification" (p. 262). 



Extraordinary homoeomorphs occur among the lobate terebratulids {Pygofe, Antinomia, 

 Pygites) and these are set forth by Buckman in his paper of 1906. 



When Hall and Clarke prepared their monograph, the perplexing feature of homoeomorphy was 

 not clearly understood, consequently some genera were made on the basis of external form alone. 

 It has been found in the present work that no one character, either internal or more especially 

 external, can be relied upon in the identification of a genus, but that a genus must be character- 

 ized by a combination of these features. Among the orthids there have been convergences either 

 toward other orthid families or toward the external form of other groups. Productorthis of the 

 early Ordovician may be cited as a conspicuous example in its resemblance to the productids of the 

 late Paleozoic. There are three genera that ape the external form of Skenidium (Skenidioides, 

 Mystropkora, Hesperorthis? (merope)), and no fewer than seven have the external form of Piono- 

 dema (Mimella, Hemipronites, Doleroides, Deltatreta (some species)), Finkelnburgia, Schizopho- 

 rella, and early Schizophoria). _ . 



Of interest in this connection is the accelerated development of the Ordovician Clitambonitids 

 of European Russia. Early species took the outside features of Strophomena, as in Gonambonites, 

 and some of these later aped forms of Hebertella. Hemipronites is much like a Pionodema, and 

 Vellamo has essentially the form of Hesperorthis. This diverse development occurred in a relatively 

 short time, and the whole stock perished with the Ordovician. 



"S. S. Buckman, Homitomorphy among Jurassic Brachiopoda. Proc. Cotteswold Nat. Field Club, vol. 13, 1901, 



^^' Brachiopod Homoeomorphy: Pygofe, Antinomia, PygiUs. Quart. Jour. Geol. Soc., London, vol. 62, 1906, pp. 433-454. 

 Brachiopod Homoeomorphy: Sfirijer glaber. Ibid., vol. 64, 1908, pp. 27-33. 



