BRACHIOPODA. 263 



added six species from the Carboniferous or Permo-Carboniferous of the Salt- 

 Range, and Abich, in 1888, described two forms from Armenia as varieties of 

 Streptorhynchus crenistria, Phillips, viz., S. eusarcus and S. incurvus, both of which 

 Dr. Waagen regards as belonging to Derbta. On the basis of these forms he 

 has made a conventional subdivision of the group into Camerati and Septati; 

 in the former, the dental lamellae do not extend along any part of the rostral 

 cavity; in the latter are species like D. Correana and D. robusta, where the me- 

 dian septum unites at the apex of the umbonal cavity with the dental lamellae 

 forming a minute triangular vault. This peculiarity is most conspicuously 

 developed in Derbya Correana, and is a natural result from the great height of 

 the cardinal area.* In Derbya crassa and others of the Septati where the rostral 

 cavity has become largely filled up by testaceous matter, a slight abrasion of 

 the beak shows this union of the septum with the dental lamellae. 



On the internal surface of a brachial valve of Productus Nebrascensis, from 

 the upper Coal Measures, near Kansas City, Missouri, there occurs a little 

 group of twelve minute pedicle-valves, all closely affixed to their host by their 

 entire external surface. With but two exceptions, the members of this cluster 

 are oriented alike, having their hinge-lines parallel to the hinge-line of the 

 Productus, but their anterior margins directed toward the posterior margin of 

 the latter. Three of the individuals show a broad cardinal area, and deltidium, 

 and bear a conspicuous median septum in the muscular area, thus having the 

 characters of Derbya. Their gregarious occurrence and small size are evi- 

 dences of immaturity. Tie frequent distortion of mature pedicle-valves of 

 Derbya, and the occasional retention of a cicatrix at the apex, are evidences 

 of an adherent condition in early stages of growth, which are confirmed by this 

 interesting specimen, of which an illustration is given upon Plate XI b. 



Two interesting new forms of this genus are described and illustrated in 

 this volume, viz., Derbya Broadheadi and D. Bennetti, from the Upper Carbon- 

 iferous limestone of Missouri and Kansas, the former a large species with but a 

 moderately high area, the latter a smaller form having much the expression of 

 a Meekella, with very high area, and an extremely high internal septum. 



* See figure on Supplementai'y plate. 



