ORDER I HY POPARIA 625 
and Upper Cambrian and Ordovician rocks of Bohemia, Scandinavia, Great Britain, 
Spain, and America. 
Microdiseus, Emmons (Fig. 1281, B). Cephalon and pygidium often showing their 
segmented nature by annulations on their axes; thorax with three or four segments. 
Cambrian. 
Family 2. Harpedidae. Barrande. 
Cephalon large, with a broad marginal expansion or limb; glabella short and pro- 
minent. Free cheeks ventral, continuous ; suture marginal, following the outer edge of 
limb. Paired simple eye-spots or ocelli, single or double, at the distal ends of well-marked 
eye-lines on the fixed cheeks, extending outward from the glabella. Thorax of from twenty- 
five to twenty-nine segments, with long grooved pleura. Pygidium 
(‘in Harpes) very small, composed of but three or four segments. 
Ordovician to Devonian. 
Harpes, in many respects, is one of the most interesting 
genera of Trilobites, being very unlike other forms. The broad 
hippocrepian pitted limb of the cephalon has its counterpart in 
Trinucleus and Dionide, although less well developed in these 
genera. The head is also relatively longer and larger, both 
features being decidedly larval. It is the only family known in 
which functional visual spots or ocelli are situated on the fixed 
cheeks. The young Trinucleus has similar ocelli. The great Fic. 1284. 
number of free segments in Harpes is another primitive — Harpes ungula,  Stern- 
3 x <a . P berg sp. Ordovician ; 
character, and the cephalon still remains larger than the thorax johemia (after Barrande). 
and pygidium. . 
Harpes, Goldfuss (Figs. 1281, C, 1284). Principally Ordovician and Silurian, 
although a few Devonian forms are known. 

Family 3. Trinucleidae. Barrande. 
Cephalon larger than the thorax or pygidium ; genal angles produced into spines. 
Free cheeks continuous, almost wholly ventral, carrying the genal spines ; suture marginal 
or submarginal. Paired sinuple eyes or ocelli generally absent in adult forms ; compound 
eyes wanting. Thoracic segments five or six, with grooved pleura. 
Pygidium triangular; margin entire; axis with w number of 
annulations ; limb 
grooved.  Ordo- 
vician and Silurian. 
Trinucleus, 
Lhwyd (Figs. 
1281, D, 1285). 
Cephalon with a 
\\ broad, regularly 
\ pitted border, pro- 

Fic. 1286. 
Fic. 1285. Fic. 1987. duced behind into 
Ampyx nasuta, ; : sale 
Trinucleus — Goldfussi, Dalman. Ordo- Ampyx Portlocki, Barr. Ordo- long genal spines ; 
Barr. Ordovician (Et. D); vician ; Pulkowa, vician (Et. D); Leiskoy, Bohennia. 
glabella prominent ; 
thorax of six seg- 
ments; axis narrow; pygidium triangular, wide, short. Ordovician; Europe and 
North America. 
Dionide, Barr. Like Trinucleus, but with irregularly punctate border, and larger 
pygidium. Ordovician ; Europe. 
Ampyz, Dalman (Figs. 1286, 1287). Thorax and pygidium resembling Trinucleus ; 
VOL. I 28 
Wesela, Bohemia, 1/3. Russia. 1/3. 1/, (after Barrande). 
