Raymond — Common Devonian Brachiopods. 287 



Figure 4. — Orthothetes 

 chemungensis var. pecti- 

 nacea Hall ; dorsal valve, 

 showing earlier plications. 

 xl2. 



exactly circular, biconvex, and smooth. It is. -35 min in length, 

 but varies from that size to -43 mm . 



Introduction of New Strice. — The 4 



shell of the varietv pectinacea has, up 

 to a length of 1 to l'2 mm , from 13 to 15 

 sharp, simple striae separated by spaces 

 which are wider than the striae. There 

 then appear from 4 to 6 new striae in 

 the middle of the front, one implanted 

 in each interspace in that region (figure 

 4). Specimen No. 1, on Plate XV, 

 Row 1, shows the shell with the original 

 striae. Specimen No. 2 shows 5 striae 

 implanted in front. Later, more are 



implanted until there is one between each pair of the orig- 

 inal striae. The next step is the appearance of striae in pairs, 

 one on each side of each of the secondary striae. (See speci- 

 mens JSTos. 8 and 10, Row 1, Plate XY.) At a still later stage 

 more pairs are added, one on each side of those next previous 

 to appear. In the variety arctistriatus, the method of develop- 

 ment is the same, but the resulting appearance is somewhat 

 different. In the earliest plicated stages there are from 15 to 

 19 sharp striae. New striae appear as before, but come in at 

 earlier stages, thus covering the surface of the valve more 

 completely and giving a more uniform appearance to the striae. 

 (Compare No. 6, Row 1, Plate XV, with No. 6, Row 2, and 

 the last specimens in each row.) 



Figure 5. — Orthothetes chemungensis Conrad ; specimen retaining both 

 valves and showing pedicle tube and simple plications, x 16. 



Figure 6. — The same species ; ventral valve, showing pedicle tube which 

 is somewhat broken at top, growth lines, and striae, x 16. 



Orthothetes bellulus Clarke. 

 (Plate XV, Eow 3.) 

 13th Ann. Kept. N. Y. State Geol., pp. 176, 187, pi. iv, figs. 2-4. 



This species was described by Clarke from specimens found 

 in the Marcellus, but the Hamilton forms from the Canandaigua 



