VI. — The Development of Terebeatalia obsoleta, Dall. 

 By Chaeles E. Beecher. (With Plates II and III.) 



Fischer and CEhlert® have given a full account of the development 

 of the brachial supports in Terehratella dorsata and Magellania 

 venosa, from Terra del Fuego. This work, together with that of 

 Friele,'^ and Deslongchamps,^ on the northern species Macandrevia 

 cranium and DalUna septigera^ and the equatorial species Muhl- 

 feldtia sanguinea, constitute nearly all that is known regarding the 

 metamorphoses taking place in the brachial supports during the 

 growth of an individual belonging to the higher genera of the Tere- 

 bratellidse. 



It is of interest to add another species to this list, especially as it 

 represents a northern form, the development of which has not 

 hitherto been studied. This form offers, moreover, some additional 

 features for comparison, and two very early stages have been dis- 

 covered which are both of genetic value. 



The material for this work has kindly been furnished by 

 Dr. William PI. Dall of Washington. The specimens were dredged 

 by the U. S. Steamer Albatross, in 113 fathoms, at Station 2984, off 

 Cerros Island, Lower California. In a report on some shells from 

 this expedition, by Dr. Dall,^ this brachiopod was described as Tere- 

 hratella occidentalism var. obsoleta. Subsequent study, however, 

 has led him to consider it as a distinct species, T. obsoleta (Plate II, 

 figures 6-9), and this determination is here adopted. A comparison 

 of the two forms shows that T. occidentalis is much wider and less 

 convex, the plications stronger, and the valves heavier. In the 

 previous paper it was stated that Terebratalia obsoleta was morphi- 

 cally equivalent to Terehratella dorsata, and that, during growth, 

 both went through different series of transformations to reach this 

 final equivalent type of structure. It is now proposed to illustrate 

 and describe in greater detail these metamorphoses of T. obsoleta. 

 The generic homologies and differences are discussed in the paper 

 already mentioned. 



The earliest stage observed in this sjjecies, Plate II, figure 10, has 

 a length of .3'""\ It is comparable to an early stage of Cistella 

 neapolitana described by Kovalevski.* Both agree in having an 



* See C. E. Beecher, Development of the Brachiopocla, Pt. II, figures 15, 16. Am. 

 J. ScL, xliv, 1892. 



