Raymond — Common Devonian Brachiopods. 299 



The difference between the varieties pectinacea and arcti- 

 striatus of Orthothetes chemungensis is a good example of the 

 effect of acceleration in the development of certain characters. 

 The steps in the development of the two are exactly the same, 

 but because the strige are introduced at an earlier stage on one 

 than on the other, the shells differ greatly in appearance. 



Productidw. — The facts in the development of Chonetes do 

 not seem to support the idea put forward by Hall and Clarke 

 that Chonetes might be descended from Plectambonites, a shell 

 which in many respects much resembles Chonetes. The early 

 neanic stages have an outline which is much the same as that of 

 Rafinesquina and Stropheodonta. The resemblance of the 

 nepionic shell to Tripleoia has already been referred to, and 

 this, with the other characters, relates the shell to the Strophd- 

 menidse. It is clearly a transition form between the latter 

 family and the Productidse. 



Paleontological Laboratory, 

 Yale University Museum, 

 January 6, 1904. 



EXPLANATION OF PLATES.* 



Plate XII. . 



Pholidops oblata Hall. 



Eows 1 and 2. — Series of dorsal valves, x 2. 

 Eows 3 and 4. — Series of ventral valves, x 2. 



Plate XIII. 



Row 1. — Partial series of Stropheodonta perplana Conrad; ventral valves. 

 Specimens Nos. 3 — 9 show well the mucronate cardinal extremities of the 

 adolescent individuals. 



Rows 2-4. — Series of Stropheodonta incequistriata Conrad; ventral valves, 

 showing the shells in neanic, ephebic, and gerontic stages. 



Plate XIV. 



Chonetes scitulus Hall. 



Rows 1 and 2. — Series of ventral valves, x 2. 



Rows 3 and 4. — Series of dorsal valves : interior view, x 2. 



Specimens Nos. 1 and 2 of Row 1, and Nos. 1 and 2 of Row 3, show the 

 ventral sinus and dorsal fold of the very young stages. Most of the speci- 

 mens in Row 4 show the brachial scars. 



Plate XV. 



Row 1. — Partial series of Orthothetes chemungensis var. pectinacea Hall; 

 dorsal valves, x 2. 



Row 2. — Partial series of Orthothetes chemungensis var. arctistriatus Hall; 

 dorsal valves, x 2. 



* Unless otherwise stated, the figures are natural size. 



