150 STUDIES IN EVOLUTION 



represented by spines QArges, Terataspisy Further progres- 

 sion of these changes is shown in Acidaspis. 



Family IX. Lichadid^ Barrande. 



Dorsal shield generally large and flat, with granulated test. 

 Cephalon small, not more than one-fourth the entire length; 

 genal angles spiniform. Free-cheeks separate; sutures extend- 

 ing from the posterior margin obliquely inward to the eyes, and 

 then almost directly forward, cutting the margin separately. 

 Glabella broad, with a large, often tumid, central lobe and from 

 one to three side lobes. Eyes not large. Thorax with nine or 

 ten segments and grooved and falcate pleura. Pygidium large, 

 flat, commonly with toothed or notched margin corresponding 

 to the pleural grooves; doublure very broad. 



Ordovician to Devonian. 



Including the genera and sub-genera Lichas Dalman, Arcti- 

 nurus Castelnau, Arges Goldfuss, Cerafolichas Hall and Clarke, 

 Conolichas Dames, Dicranogmus Corda, HomoUchas Schmidt, 

 Hoplolichas Dames, Leiolichas Schmidt, Metopias Eichwald, 

 Oncholichas Schmidt, Platymetopus Angelin, Terataspis Hall, 

 Trochurus Beyrich, and Uralichas Delgado. 



Most of the forms of this family are above the average size 

 of trilobites, and several species are among the largest of the 

 class. They are all thin-shelled, and were loosely articu- 

 lated, so that entire specimens are extremely rare. 



A great diversity is shown in the form and lobation of the 

 glabella. In Lichas (sens, str.), Platymetopus^ and Leiolichas 

 the anterior lobe dominates and is continuous with the axis. 

 In Hoplolichas and HomoUchas the lateral lobes are strongly 

 defined, and each is nearly equal in size to the central lobe. 

 Dicranogmus, Oncholichas, Conolichas, Bletopias, Arctinurus, 

 and Arges show the lateral lobes divided transversely into 

 two or three smaller ones. Lastly in Ccratolichas, and more 

 especially in Terataspis, the central lobe becomes a promi- 

 nent ovoid or globular extension. These variations evidently 

 indicate differences in the relative development of the append- 



