344 C. K Beecher — Development of the Brachiopoda. 



The Protegulum. 



The first important observation to "be noted is, that all 

 brachiopods, so far as studied by the writer, have a common 

 form of embryonic shell, which may be termed the protegulum .*" 

 The protegulum is semicircular or semielliptical in outline, 

 with a straight or arcuate hinge line, and no hinge area. A 

 slight posterior gaping is produced by the pedicle valve being 

 usually more convex than the brachial. The modifications noted 

 are apparently due to accelerated growth, by which characters 

 primarily nealogic 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 generaf 

 representing nearly all the leading families of the class, its gen- 

 eral presence may be safely assumed. In size it varies in 

 different genera and species. The range is from "05 to *60 mm . 

 A similar range in the prodissoconch of pelecypods has been 

 noticed by Dr. Robert T. Jackson. The protoconch of cephal- 

 opods and gastropods also varies greatly. In all these classes, 

 the size of the initial shell has no special 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 cor- 

 neous and imperforate. In all probability it is the same for 

 the entire class, whether among the corneous and phosphatic 

 linguloids and discinoids, or the terebratuloids and other forms 

 having carbonate of calcium shells. Professor E. S. Morse, in 

 describing the early stages of Terebratulina,:): 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 Paramsecia." Similarly, in the genus Cis- 

 tella according to Kowalevski :§ " En meme temps la coquille 

 se forme, par suite du depot sur la cuticule chitineuse des 

 minces couches de calcaire, dans lesquelles on ne voit point 

 encore les perforations tubulaire." Previous to this stage, 



* From 7rpw, early, and reyog, a covering. 



f Atretia (Cryptopora), Chonetes, Cistella, Conotreta, Crania. Craniella, Disciua, 

 Discinisca, Glottidia, Gwynia, Kraussina (Megerlina), Laqueus, Leptaena, Lingula,. 

 Lingulops, Linnarssonia, Liothyrma, Magellania (Macandrevia), Martinia, Muhl- 

 feldtia, Obolus? (Ehlertella, Orbiculoidea, Orthis group, Pholidops, Productella, 

 Rhynchonella (H.emithyris), Schizambon, Schizobolus, Schizomania, Schizotreta, 

 Spirifer, Streptorhynchus (Orthotetes), Stropheodonta, Strophomena, Terebratella, 

 Terebratulina, Tkecidium (Lacazella), Trematis, Tropidoleptus, Zygospira. 



% Embryology of TerebratuliDa. Mem. Boston Soc. Nat. Hist., vol. ii, p. 257, 

 ride figures 68, 76, pi. viii. 1873. 



§ Developpement des Brachiopodes, Kowalevski. Analyse par MM. (Ehlert et 

 Deniker, pp. 65, 67. 1883. 



