154 STUDIES IN EVOLUTION 



The cephala of Areia and Placoparia have many resemblances 

 to Conocoryphe, but the fixed-cheeks bear the genal angles 

 and spines, while in the latter genus they are on the free- 

 cheeks. In both families the free-cheeks are narrow and 

 marginal, and the eyes are absent or rudimentary. Both 

 these characters are decidedly larval. Other primitive and 

 larval features belonging to the Encrinuridse are the eye- 

 lines in Cybele and Encrinurus, the undefined and expanded 

 termination of the glabella in Dindymene and Encrinurus, 

 and the pentamerous head axis in all but Dindymene, in 

 which the four anterior lobes or annulations are obsolete. In 

 Encrinurus the eye-line in meeting and joining the anterior 

 lobe of the glabella sometimes gives the appearance of an 

 extra lobe, as in Ogygia and Paradoxides. 



Family XII. CALYMMENIDJS Brongniart. 



Cephalon somewhat wider than long. Fixed-cheeks large; 

 genal angles rounded or produced into spines. Glabella nar- 

 rowing anteriorly. Free-cheeks long, separate, usually with 

 a free rostral plate between the anterior extremities. Sutures 

 extending from just in front of the genal angles, converging 

 anteriorly, and cutting the margins separately. Eyes small; 

 visual surface seldom preserved. Thorax of thirteen segments, 

 with grooved pleura. Pygidium of from six to fourteen seg- 

 ments; axis tapering. Ordovician to Devonian. 



Including the genera and sub-genera Calymmene Brongniart, 

 Brongniart ia Salter, Burmeisteria Salter, Calymmenella Ber- 

 geron, Calymmenopsis Munier-Chalmas and Bergeron, Dipleura 

 Green, Homalonotus Koenig, Koenigia (= Homalonotus) Salter, 

 Pharostoma Corda, Plcesiacomia Corda, Ptychometopus Schmidt, 

 and Trimerus Green. 



The genera of this family naturally cluster around the two 

 leading ones, Calymmene and Homalonotus. Closely related to 

 the first are Ptychometopus, Pharostoma, Calymmenopsis, and 

 Calymmenella, all agreeing in having the glabella well defined 

 and marked by furrows or indentations at the sides, corre- 

 sponding to its segmental nature. 



