•70 SYSTEMATIC ARRANGEMENT 



1. O. (jihbusiis : Scuto capitis inter suturam facialem et umbonem tuberculo transverso signato ; 

 rhachi coi-poris quaterdecies annulata, caudae quinquies. Long. 1". Table III, Fig. 9. 



Bef. — Tr. truncatus, Brunn. N. Act. Hafn. \, 391. Modeer in Berl. Gesellsch. Schrift. 

 vi, Table II, Figs. 3-.5. Entom. gibhosm, Wahlenb. N. A. Ups. viii, 39. 12, Table I, 

 Fig. 4. Brongn. Cr.foss. 35, PI. Ill, Fig. 6. Schloth. Nachtr. ii, 26. 4. 36. 18. 

 Dalm. Palaad. 56. 4. 74. 4. Boeck, Mag. f. Nat. i, 24. Emmr. Dissert. 45. 1. 

 Milne Edw. Crust, iii, 343. 4. Hising. Leth. suec. 19, Tab. IV, Fig. 3. 

 Cephalic shield four times as broad as it is long, th& axis remarkably narrow ; an 

 elevated elliptical prominence both to the left and right at the anterior extremity, the pro- 

 minence extending as far as the facial suture.* The number of joints in the axis of the 

 body is fourteen ; the lobes of the first joints are twice as broad as the axis ; those of the 

 last only a little broader. 



Caudal shield semicircular; the axis five-jointed, with an anterior margin of articu- 

 lation ; the lateral portions flat, without rays, the margin rather reflexed. 

 Locality. — In the alumslate and stinkstein of Andrarum. 



Remarks. — 1. The cheek-sliields of the head are absent in all the older descriptions and figures, 

 being always broken off. They are often present, however, near the other remains, so that there can 

 hardly be a doubt about theii- existence. I counted fourteen body joints in the impressions of young 

 and perfect indi\iduals. 



2. Asaph, tetragonocephahis (Green, Sill. Am. Jo. vol. xxv, p. 336 ; Emm. Diss. 46. 4 ; Milne 

 Edw. Cr. iii, 330) is so similar to 01. gibbosiis, that they are liable to be confounded with one another ; 

 indeed I was not able to discover satisfactory specific distinctions in the plaster model which I examined 

 at Berlin. I counted in this specimen fourteen body-rings, and certainly three caudal rings, but the 

 latter were indistinct and imperfect. The species was found in the alumslate of Lockport. 



3. Boeck, in Kielhaus's Gcea Norwegica (see Leonhard and Bronn, Zeitshr. 1841, p. 727), has 

 characterized two species nearly related to 01. gibbosus, which I am not acquainted with, and therefore 

 give them here according to his statement. 



O. alatus (/. c. No. 38) is nearly related to 0. gibbosus, but the glabella (which is the only part 

 known) is proportionably much narrower, and the transverse prominence which issues from its anterior 

 extremity does not extend in so straight a #iue, but is produced more backwards. 



O. latus (1. c. No. 39) is much larger than 01. gibbosus, and the piece (probably the space) between 

 the glabella and eyelid is considerably broader. 



I do not think such differences in imperfect fragments can justify us in founding new species. 



4. Emmerich's Pur. acuminatus (Dissert. 46. 2), which is said to be distinguished from 01. gibbosus 

 by a more developed angularity of the facial suture before the eye, and by a bending of it inwards at 

 the posterior extremity, also appears to me merely to indicate an individual difference caused by differ- 

 ence of preservation, as this is easily accounted for in the impressions of tender parts. 01. gibbosus in 

 other respects varies, like its allies, very considerably in size, according to the difference of age ; I have 

 seen specimens of Wi" length, and others scarcely !^". 



2. O.forficula. 



Sars, Isis, 1835, 333, Tab. VIII, Fig. 1. Milne Edw. Crust, iii, .343. 1. 



According to the figures, this species most nearly resembles Paracl. spitiulosiis in the 

 iiabit of the head {im Habitus des Kopfes), but has a glabella somewhat broader anteriorly, 

 and divided by two furrows into three nearly equal parts. A slight longitudinal impression 

 appears at the anterior of these, and on the third there is a small tubercle. Behind it the 



* By its position this prominence justifies the assumjrtion that a small tentacle issuing from the 

 glabella has existed beneath it in a cavity of the shield. 



