NATURAL CLASSIFICATION OF THE TRILODITES 137 



Family II. Hakpedid^e Barrande. 



Cephalon large, margined by a broad expansion or limb; 

 glabella short and prominent. Free-cheeks ventral, continu- 

 ous; suture marginal, following the outer edge of the 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. 



Cambrian to Devonian. 



Including Harpes Goldfuss, Harpina Nov^k, and Harpides ? 

 Beyrich. 



The genus Harpes presents considerable variation in the 

 lobes of the glabella. H. ungula Sternberg shows the full 

 number of five lobes, but in some species, as H. d^Orbig- 

 nyianum Barrande, the structure is like Cyphaspis, with 

 separate basal lobes. Arraphus Angelin was apparently 

 based upon a specimen of Harpes denuded of the pitted 

 border. Harpides Beyrich is imperfectly known, but seems 

 to belong here. The ocular ridges and tubercles on the 

 fixed-cheeks, the broad limb, the glabella, and the narrow 

 weak thoracic segments are all in accord with Harpes, though 

 in other features it has affinities with the Conocoryphidse. 



In many respects Harpes is one of the most interesting 

 genera of trilobites, since it is so unlike other forms. The 

 broad hippocrepian pitted limb of the cephalon has its 

 counterpart in Trinucleus and Bionide, although not so well 

 developed in these genera. The cephalon is also comparatively 

 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 Trinu- 

 cleus has similar eye-spots, or ocelli. The great number of 

 free segments in the Harpedidse is another primitive char- 

 acter, although the cephalon (in Harpes) still remains larger 

 than the thorax and pygidium. 



