326 PALEONTOLOGY OF NEW YORK 



Waaqen's term for this group, Gastropegmata (or Craniacea) may therefore 

 prove to be equivalent to each of these other two divisions. 



The great gulf which has seemed to exist between the Inarticulate or 

 Lyopomatous, and the Articulate or Arthropomatous divisions of the Class 

 Brachiopoda ; those without teeth and those with teeth ; those with a largely 

 corneous shell, and those whose shell is essentially calcareous, is not yet fully 

 spanned at many points. 



These divisions were based upon the study of living brachiopods in which 

 all the characteristic differences are pronounced and fixed. We naturally ex- 

 pect to find, however, among the early brachiopods, in which the adjustment 

 of the organism to its conditions was highly sensitive, that the oscillation and 

 specialization of characters has been very rapid. The development of articulat- 

 ing processes has already been noticed among the linguloids, in Barroisella, 

 ToMASiNA and Trimerella, among the oboloids in Spondylobolcs, and among 

 the siphonotretoids in Trematobolus. It is known that the shell of many inar- 

 ticulates is almost wholly calcareous, as in the TniiiEitELLiDjE and all of the so- 

 termed Gastropegmata. The alteration in the nature of the shell-substance from 

 protoconch, or its exemplar, Paterina, which appears to be wholly or essentially 

 corneous, to the typical articulate brachiopod, in which the corneous sub- 

 stance is reduced to a thin epidermal film, is a gradual process whose various 

 stages are well understood. In Obolella, Elkania, and the early forms of Lin- 

 GULA, the deposition of calcareous salts in the shell was already advanced, these 

 layers alternating with thinner layers of corneous substance. The gradual and 

 eventual predominance of the calcareous shell-matter along both of these lines 

 of development is seen in the ponderous Triraerellids of the later Silurian. 

 The graduation of the corneous Paterina (Kutorgina Labradorica, var. Swanton- 

 ensis) through Kutorgina Labradorica, and into the true calcareous Kutorginas 

 (A', cingulata, K. Whitjieldi), is similar evidence. In Kutorgina Latourensis, Mat- 

 thew described a minute tooth on either side of the pedicle-opening,* and it 

 has been stated that K. cingulata shows faint traces of articulating processes at 



* lUustrations of the Fauna of the St. John Group, No. 3, p. 42. 1885. 



