SUBCLASS I 



TRILOBITA 



711 



Eodiscus Matthew (Microdiscus Salter non Emmons) (Fig. 1357, B). Glabella short, 

 occipital ring spined. Lower and Middle Cambrian ; Europe and North America. 



Goniodiscus Raymond. Glabella long, occipital ring obtusely pointed. Type, 

 Microdiscus lobatus Hall. Lower Cambrian ; North America. 



Family 3. Shumardiidae Lake. 



Hypoparia similar to the Agnostidae in size and in structure of the cephalon, but 

 with small, stronyly segmented pygidium, and six segments in the thorax. 



Shthmardia Billings. Glabella proniinent, expanding toward the front. Lower 



Ordovician ; Europe and North America. 



Family 4. Harpedidae Corda. 



Hypoparia loith very large head shield and small pygidium. Free cheeks ventral, 

 sutitre marginal. Tliorax with numero^is segments {seventeen to twenty-nine). Cephalon 

 with hroad pitted hrim. Ordovician to Devonian. 



The broad hippocrepian pitted hvhn of the Harpedidae has its counterpart in 

 Gryptolithus and Dionide, although less well developed in those 

 genera. The head is also relatively longer and larger, both 

 features being decidedly larval. The functional visual spots or 

 ocelli, situated on the fixed cheeks, are found only in this 

 family and in Tretaspis arid in the young of Gryptolithus. Thu 

 great number of thoracic segments is another primitive 

 character, and the cephalon is larger than the thorax and 

 pygidium. 



Harpes Goldfuss (Fig. 1357, G). Hypostoma somewhat 

 pentagonal, angular in outline. Silurian and Devonian ; 

 Europe and North America. 



Eoharpes Raymond {Harpina Novak) (Fig. 1360). Hypostoma 

 oval in outline. Ordovician ; Europe and North America. 



Fig. 1360. 



Eoharpes ungula (Stern- 

 berg). Ordovician ; 

 Bohemia, l/i (after Bar- 

 lande). 



Family 5. Trinucleidae Emmrich (Gryptolithidae Angelin). 



Hypoparia with large cephalon and small pygidium. Free 

 cheeks ventral, carrying the gcnal spines. Gephalon with a pitted, 

 hrim. TJwracic segments few {five or «?";<■). Ordovician. 



Gryptolithus Green {Trinucleus Murch.^mrs) (Figs. 1346, 1357, 

 D ; 1361). Central portion of cephalon divided by the dorsal 

 furrows into three prominent portions. No ocelli in adult. 

 Ordovician ; Europe and North America. 



Trinucleus Murch. (restr.). Glabella obovate, with two pairs 



;ryptoiithus (jMfussi ^f ^^^^ glabellar furrows. Pits on brim set in deep radiating 



furrows; no ocelli or eye-lines. Ordovician; Europe and N orth 



America. Type T. fimbriatus Murch. 



Tretaspis M'Coy. Glabella spherical in front, conical behind, with two pairs of 



strong glabellar furrows ; ocelli and eye-lines present. Ordovician ; Europe and 



North America. 



Dionide Barrande. Similar to Gryptolithus, but with an irregularly pitted border 

 and a large pygidium. Ordovician ; Europe. 



Fig. 1361. 



(Barr.). Ordovician (Etage 

 D) ; Wesela, Bohemia. 

 X i/i. 



