DEVELOPMENT OF THE BRACHIOPODA 231 



modifications noted aie apparently due to accelerated growth, 

 by which characters primarily neanic become so advanced in 

 the development of the individual as to be impressed finally 

 upon the embryonic shell. This feature is well shown in the 

 development of Orbiculoidea and Discinisca, and is reserved 

 for discussion under these genera. 



As the protegulum has been observed in about forty genera * 

 representing nearly all the leading families of the class, its 

 general presence may be safely assumed. In size it varies 

 in different genera and species. The range is from .05 to 

 .HI) mm. A similar range in the prodissoconch of pelecypods 

 has been noticed by Dr. Robert T. Jackson. The protoconch 

 of cephalopods and gastropods also varies greatly. In all 

 these classes the size of the initial shell has no si3ecial 

 relation to the mature form, and it seems to have little 

 significance in related genera or species. 



The structure of the protegulum has been described as 

 corneous and imperforate. In all probability it is the same 

 for the entire class, whether among the corneous and phos- 

 phatic linguloids and discinoids, or the terebratuloids and 

 other forms having carbonate of calcium shells. Professor 

 E. S. Morse, in describing the early stages of Trrebratulina,f 

 says : " A heart-shaped corneous shell is formed even at this 

 early stage, for in several cases I met with it where the softer 

 portions had been removed by Paramtecia." Similarly, in 

 the genus Cistella according to Kovalevski:| "En meme 

 temps la coquille se forme, par suite du d^pot sur la cuticule 

 chitineuse des minces couches de calcaire, dans lesquelles on 



* Atretla {Crj/ptopora), Clwnetes, Cistella, Conotreta, Crania, Craniella,Discina, 

 Discinisca, Glottidia, Gwijnia, Kraussina (^fegeTlina), Laqueus, Leptcrna, Lingula, 

 Lingulops, Linnarssonia, Liothyrina, Magellania (Macandrevia), Marlinia, Miihl- 

 fddlia, Oholus 1 (Ehlertella, Orbicidoidea, Orthis group, Pholidops, Productella , 

 Rhynchonella (Ilemithyris), Schizamhon, Schizobolus, Schizocrania, Schizotreta, 

 Spirifer, Streptorhijnchvs (Orthothetes), Stropheodonta, Stropliomi'na, Terebratella, 

 Terebratulina, Thecidium, (Lncazella), Trematis, Tropidoleptus, Zi/gospira. 



t Embryology of Terebratulina. Mem. Boston Soc. Nat. Hist., II, 257, vide 

 figs. 68, 76, pi. vi!i, 1873. 



{ Developpement des Brachiopodes, Kovalevski. Analyse par MM. CEhlert 

 et Deniker, 6."), 67, 1883. 



