392 Transactions. 



Art. XLII. — Brachiopod Genera : The Position of Shells with Magaselliform 

 Loops, and oj Shells with Bouchardiform Beak Characters. 



By J. Allan Thomson, MA., D.Sc, F.G.S. 



[Read before the Wellington Philosphical Society, 28th October, 1914.] 



The genus Magasella was founded in 1870 by Dall* for shells for erly 

 included under Terebratella, but distinguished from Terebratellae proper by 

 the possession of a high septum and by the fact that the reflected portion 

 of the loop forms a ring behind the upper portion of the septum. 



It was subsequently discovered that during growth Terebratella passes 

 through a stage exactly comparable to Magasella, and that Magellania 

 (M. venosa) passes through stages comparable to Magasella and Terebra- 

 tella^ ; and, further, that a large number of species that had been described 

 as Magasellae were in reality only the young of known species of Terebra- 

 tella and Magellania. This proved to be the case with Terebratella evansi 

 Davidson, which Dall had chosen as the type of his genus Magasella, for 

 this species, originally described from Lyall Bay, Wellington, is a young 

 form of the common New Zealand shell Terebratella sanguinea Leach. 

 According to a strict application of the law of priority, then, Magasella is 

 a synonym of Terebratella. As there are, however, shells with Magaselli- 

 form loops which are undoubtedly adult, a generic name, embodying Dall's 

 intentions is necessary, and at first sight it appears to be a case in which the 

 law of priority might be set aside with advantage. This course was followed 

 by Deslongschamps4 who proposed the South Australian Recent shell 

 Terebratella. cumingi Davidson as the genotype, in place of T. evansi. If 

 all shells with Magaselliform loop characters could be placed in Magasella 

 thus emended, this course would have no other objection than the lack of 

 finality that must always attach to any alteration of the law of priority. 

 As will be shown below, however, Magaselliform loop characters may be 

 attained independently in more than one stock, and a different procedure 

 becomes necessary. Bud. man has already pointed out this polygenetic 

 origin of loop characters in the case of shells hitherto placed under Magel- 

 lania and Terebratella.^ 



'' It may be remarked that as species of Magellania pass through a 

 Terebratelliform stage in their ontogeny, pointing to a Terebratelliform stage 

 of phylogeny, the names Magellania and Terebratella do no indicate proper 

 generic divisions, but nark the stage of loop-development attained. It is 

 therefore more likely that certain Magellaniae are really Terebratellae which 

 have attained the Magellaniform loop, and that certain Terebratellae are 

 really Magellaniae which have not yet lost the Terebratelliform loop. This 

 means that a rearrangement of these genera to correspond with vertical lines 

 of descent instead of to indicate horizontal lines of developmental stages 

 may be anticipated. It is known that among Terebratuloid forms the shape 

 (test and ornament) may change considerably while the loop remains without 

 much modification ; and, on the other hand, that the loop may change 

 considerably while the shape remains little altered." 



* Am. Journ. Conch., vol. 6 (1870), p. 97, fig. 18. 



t P. Fischer and D.-P. Oehlert, Bull. Soc. Hist. nat. d'Autun, t. 5 (1892), pp. 254-334. 

 % Etudes critiques sur des Brachiopodes, &c. (1884), p. 204. 



§ Wissensch. Ergobn. Schwed. Siidpolar-Exped., 1901-3. bd. iii, lief. 7 (1910), 

 pp. 21, 22. 



