DECHENELLA. HARPES. 29 



of the tail has fewer rings and these are distinct to its extremity. D. verticalis 

 (Burm.) 1 has a wider head, a less distinct border, and its eyes are more centrally 

 situated. D. elegans (Miinst.) 2 is much smaller and very unlike it in details. 

 D. Haldemanni (Hall) 3 is perhaps the nearest to it, but its border is wider, the groove 

 within itlinear, the eye further from the glabella, which is narrower, the cheeks 

 much shorter, the hind margin less oblique, and the rings of the tail much less 

 numerous. D. marginalis (Conrad), 4 has only about half as many rings in the 

 tail, and D. striata, Stanier, 5 has a prominent striated border, a shorter and 

 differently marked glabella, and much flatter cheeks. 



The heads of one or two of the Bohemian species of Proetus present approxi- 

 mations to this genus, from which they differ altogether in the shape of the tails. 

 P. Astyanax, Corda, 6 of which no pygidium is given by Barrande, is especially to 

 be noted in this relation. 



In a Russian species, D. Bomanouski described by Tschernyschew 7 the border 

 is larger, striated, and triangularly elevated ; the glabella is narrower ; and the tail 

 has a broader axis, with only sixteen rings, and is separated from the ribs of the 

 limb by a broad smooth groove. Another species, off. D. Verneuili, described 

 by the same author, has the border still more elevated. 



VI. Family. — Haepedidjj, Barrande, 1852. 



1. Genus. — Harpes, Goldfuss, 1839. 



This is the only genus belonging to its family, and may be at once distin- 

 guished by the large, flat, and porous ungulate limb of the head, the numerous 

 (twenty-five to twenty-nine) thoracic segments, and the minute tail. It extends 

 from the Lower Silurian to the Middle Devonian, and the present species is 

 its type. 



1 1846, Burmeister, ' Org. Tril.,' Eay Soc, p. 100, pi. v, fig. 9 a. 



2 1842, Miinst., ' Beitr.,' pt. 5, p. 114, pi. x, fig. 2. 



3 1861, Hall, ' Descript. of New Species, &c.,' p. 74. 



4 1887, Tschernyschew, 'Mem. Com. Geol. Buss.,' vol. iii, pt. 3, p. 12, pi. i, figs. 4—8. 



5 1887, Stanier, ' Ann. de la Soc. geol. de Belgium,' vol. xiv, p. 75, pi. iv, fig. 1. 



6 1852, Barr, ' Syst. Sil. Bohem.,' vol. i, p. 470, pi. xvii, figs. 22, 23, fit. F. 



7 1887, Tschernyschew, ' Mem. Com. Geol. Buss.,' vol. iii, pt. 4, p. 14, pi. i, fig. 10. 



