338 ME. S. S. BTTCKMAN OX BEACHIOPOD MORPHOLOGY. [Aug. I907, 



21. Beachiopod 3Iobphologt : Cincta, Eudesia, and the Develop- 

 ment of Bibs. By S. S. Buckman, F.Gr.S. (Head June 5th, 



1907.) 



[Plate XXIV.] 



The test-ornament of Brachiopods is found in three main phases — 

 smooth, ribbed, and spinous. 1 These three phases are in this 

 anagenetic sequence to one another : — in relation to its nearest allies, 

 a costate species of a given series is more advanced than a smooth 

 one of that series, and a spinose one still further than a costate. 

 There are catagenetic developments also in reverse order : — in certain 

 Productids the costate stage follows on a spinose : in Acantliothyris 

 there are certain cases of the spinose ontogenetic stage being fol- 

 lowed by a smooth. If, however, the catagenetic phases be put 

 aside for the present, it may be said that the state of external 

 ornament: — smooth, costate, spinose — indicates the position of a 

 brachiopod as more or less advanced than its fellows. 



The present paper is concerned only with two cases of the 

 development of the ribbed stage. The passage of the smooth into 

 the ribbed stage is not acquired in the same way : there are various 

 methods by which similar looking ribbed forms have been evolved 

 from similar looking smooth forms. This has a most important 

 bearing on their classification. If the phylogenetic history of similar 

 ribbed forms is demonstrably different, it is, according to modern 

 opinion, a technical error to place them in the same genus, even if 

 their interior organization be similar. If a given smooth form can 

 have developed into one of the ribbed forms but not into the other, 

 it is a technical error to unite it with the costate form into which 

 it could not develop. A case of this kind calls for discussion now. 



Dall in his ; Index * 2 says that Cincta, Quenstedt (the Terebratula- 

 numismalis-growp) is a synonym of Eudesia, King (the T.-cardium- 

 group). 3 jSow Cincta is in the smooth stage and Eudesia is in the 

 costate stage ; but, though it would be quite possible for costate 

 forms to be developed from the smooth Cincta, yet it is obvious 

 that they would not be costate forms of the Eudesia-tjipe. Cincta 

 shows the incipient development of costation ; but the costae that 

 it could develop would be of the type which produced opposite 

 carination of both valves — a carina in the one valve is opposed by 

 a corresponding carina in the other. In Eudesia, however, the 

 costae in the two valves are alternate : the carinas of one valve are 

 opposed by sulci in the other. Here is a fundamental difference, 

 which prevents the inclusion of Eudesia and Cincta in one genus : 

 it shows that they belong to entirely different series. 



1 A striate stage is sometimes interposed between the smooth and the ribbed, 

 but not always. 



2 Bull. U.S. Nat. Mus. No. 8 (1877) p. 20. 



3 The type of Eudesia is Terebratula orbicularis, Sowerby, not T. cardium, 

 Val. (Lamarck) as usually stated in text-books. 



