274 SYNOPTIC COLLECTION. 



segment which subsequently becomes the pedicle ; after- 

 wards it is fastened to the pedicle valve. Thecidium is 

 especially interesting as it is the only Brachiopod which 

 retains a true deltidium at maturity. 1 



It is instructive to note that after the pedicle is lost, in 

 several genera of this order, spines are developed for the 

 purpose of anchoring the animal. In Chonetes granu- 

 lifera Owen (No. 669), these spines are found only on the 

 cardinal margin of the pedicle valve. Productus nebras- 

 kensis Owen (No. 670), however, inherits such a weak 

 pedicle that this organ soon disappears, and spines are 

 developed over the entire surface of the pedicle valve and 

 sometimes are also found on the brachial valve. The 

 animal lies on the pedicle valve fastened by its spines. 



Vestiges of the cardinal areas and teeth are sometimes 

 present in this genus, but are often wanting. The beak 

 is high and curved, and no trace of a pedicle opening 

 remains. Many of these characters are intensified in 

 the old age stage (No. 671). The shell extends ante- 

 riorly and broadens outward at the same time. The 

 beak of the pedicle valve is bent far inward and the ped- 

 icle opening has wholly disappeared. The ribs which 

 are prominent and regular in the adult stage, partially 

 die out and become more or less irregular (see No. 671). 



Interesting specializations of structure are illustrated 

 by Orthis (= Platystrophia 2 ) (No. 672). We have here 

 large cardinal areas and perfect articulation by teeth and 

 sockets. The area of the pedicle valve has a large 

 pedicle opening or delthyrium, and that of the brachial 

 valve has a similar opening, the chelyrium. When young, 

 these openings are partially closed by plates, the deltidium 

 and chilidium. In the course of development, however, 

 the plates are resorbed so that open passages remain in 

 the adult (see No. 672). 



1 Hail and Clark, Nat. Hist. N. Y., Pal, VIII, part 2, 1894, p. 

 283. 



2 See Cumings, Amer. Journ. Sci., (4), XV, nos. 85, 86, 1903. 



