94 SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. [bull. 87. 



The external characters as expressed by both valves are manifestly nearer to Kutor- 

 gina than to any telotremate genus. * * » It should be noted, however, that the 

 youngof Choiietcs, Productus, Stropheodonta, Orthothetes, Leptpena.Plectamboniti'S, 

 and Stro])honiena, all have little or no indication of a straight hinge line, and that 

 the extension of this' nieniljer takes place during later nealogic and eplic1)i)Iic growth. 

 (Pp. 150-151.) 



By far the greatest number of Neotremata occurring' in the Lower 

 Cambrian are species of the family Acrotretiche. To the writer it has 

 always seemed strange to suppose that this family has been derived 

 through the Trematida?, but the above interpretation of the prodeitid 

 ium in Discinisca indicates that the turning of this plate posterior to 

 the pedicle at once led to holoperipheral growth in some of these early 

 forms. In some species of the Acrotretidie there is a true deltidium. 

 In Acrothele the cardinal area is flat, without any trace of a deltid- 

 ium, whereas in Acrotreta and Conotreta, which have high cardinal 

 areas, there is a narrow concave dei)ression bisecting it. These del- 

 tidia, whether convex or concave, are in all probability initiated by the 

 prodeltidium, as in the Protremata. In the family Trematidie there 

 appej^irs to be nothing homologous with the deltidium, since the plates 

 situated in the apex of the wide triangular fissure of Schizocrauia and 

 Lingulodiscina seem to be formed anterior to the pedicle and subse- 

 quent to its movement posteriorly with growth, and not posterior to 

 the pedicle, as in the Acrotretidte, These plates in the Trematidie 

 should probably be homologized with the listrium of the Discinidte. 



The complete harmony of the muscular system in the Protremata 

 and Telotremata is no evidence in itself that the latter were derived 

 from the former. The occurrence at the base of the Cambrian of very 

 ])rimitive species of the four brachiopod orders is proof that diver- 

 gence took place very early in the history of the class, and while there 

 is little knowledge of the muscles in either Iphidea, Kutorgina, or Pro- 

 torhyncha (P. f minor and P. f ambigua),the earliest genera of Atremata, 

 Protremata, and Telotremata, respectively, there is some evidence for 

 supposing them to be as in the type embryo stage of living species. 

 The high degree of specialization attained by Lingula (Atremata), as 

 exempli tied by the burrowing habit, long peduncle, and absence of valve 

 articulation, is the (;ause for their com])lex muscular system, while the 

 development of a functional hinge in the Protremata and Telotremata 

 has led to the retention of very primitive conditions or to the simplili- 

 cation and harmony of the muscles tliroughout these two orders. 



The presence of a terminal intestinal opening in the living species of 

 the Atremata and Xeotremata and its general absence in those of the 

 Protremata and Telotremata is no longer held to have phylogenetic 

 significance, as many of the Paleozoic species of the two latter orders 

 afford good evidence of such having been i)resent in the median line as 

 in living Crania.' 



' See p. 113. 



