696 



AETHROPODA 



PHYLUM VII 



The hypostoma,^ or labrum, 

 Crustaceans, and consists of a 



A 



Hypostoinas. *A, Corydocephalus jialniatus. 

 C, Ericfinurus intercostatus, side and front views 

 fafter Novak). BB, Anterior edge ; M, Middle 

 furrow ; E, Posterior furrow of tlie middle por- 

 tion ; P, Posterior- edt;e ; L, Lateral edge ; y, Pos- 

 terior wing. 



and Aulacopleura, but it may extend to the frontal border, as in Placofaria or 

 Ceraurus, or even beyond, as in Pliacops, Ampyx and ConoUchas. The entire 

 portion of the glabella which lies in front of the anterior lateral furrows, and 

 which is often somewhat enlarged laterally, is called the frontal lobe. Some- 

 times the limitation between the glabella and fixed cheeks is scarcely defined, 

 as in Illaenus and Dipleura. Most frequently, however, three pairs of grooves 

 can be distinguished in front of the neck furrow, marking the pentamerous 

 division of the glabella and the five pairs of appendages attached to the 

 cephalon. Sometimes the lateral furrows are continuous across the glabella, 

 or again, they may be directed obliquely (Triarthrus), or even form longi- 

 tudinal grooves (ConoUchas). 



is homologous to the upper lip of other 

 separate plate attached by an articulating 

 surface or line to the reflexed border of 

 the cephalic shield (Fig. 1338). 



In front of the hypostoma is a rostral 

 area sometimes partly occupied by a 

 separ-ate plate, the epistoma (lUaemis, 

 Calymene). 



The fixed cheeks are lateral extensions 

 from the glabella, to which they are 

 firmly joined, forming the centx'al portion 

 of the cephalon. They may occupy more 

 than two -thirds of the cephalon, as in 

 Conocoryphe, or may become greatly reduced, as in Lichas and Proetus. The 

 cranidium consists of the glabella and the fixed cheeks. 



The free cheeks carry the compound eyes, and are separated from the 

 cranidium by a suture. They may form (a) a continuous ventral plate, as in 

 Harpes, Jgnostus, Cryptolithus, etc. ; they may include {h) a greater or lesser 

 portion of the dorsal surface, being either entii-ely separated by the cranidium, 

 or (c) meeting, and (d) sometimes coalescing in front. They are widely 

 separated in Ptychoparia, in juxtaposition in Asaphus, and continuous in 

 Dalmanifes. 



The genal angles are the posterior lateral angles of the cephalon. They 

 may be rounded, as in Illaenus, angular, as in Goldius, or spiniform, as in 

 Cryptolithus and Dalmanites. They belong either to the fixed cheeks, as in 

 Dahnanites, or to the free cheeks, as in Illaenus, Goldius and Proetus. 



The character of the cheeks is especially influenced by the facial sutures 

 separating the free cheeks from the rest of the cephalon. They appear 

 as sharply defined lines beginning either at the posterior margin, or 

 near the genal angles, or on the lateral margins, and extend to the eyes, 

 thence around the inner margin of the visual areas, then turn anteriorly, and 

 either unite in passing around the front of the glabella or remain separate, in 

 which case the sutures terminate in the anterior margin. The position of the 

 facial sutures thus determines the relative size of the fixed and free cheeks. 

 After the death of the animal, or after moulting, the cephalic shield frequently 

 fell into pieces, dividing along these sutures. 



In most Trilobites, the existence of eyes has been demonstrated, though 



^ Brijgger, W. C, Uber die Ausbilduiig des Hypostouies bei einigen skandiuavisclieu Asapliideu. 

 Bihang K. Svensk. Vet. Akad. Handl., 1886, vol. xi. 



