488 Prof. H. H. Sivinnerton — Classification of Trilobites. 



That of Gurich is as follows : — 



Order TFJLOBITA. 



Series Oligomeria : — With few free trunk segments. 



Sub-order 1. Isopygia. Pygidium about the same size as the cephalon. 



Agnostidse, Microdiscidse. 

 Sub-order 2. Heteropygia. Pygidium distinctly smaller than the cephalon. 

 Trinucleidse, Ampycinse, Dionidae, iEglinas. 

 Series Pliomeria : — With many free trunk segments. 

 Sub-order 3. Micropygia. Pygidium very small. 



Olenellidse, Paradoxidse, Remopleuridse, Ellipsocephalidse,, 

 Harpedidse, Olenidse, Arethusinidse, Cyphaspidse. 

 Sub-order 4. Macropygia. Pygidium large, often equal to the cephalon. 

 Group a. Opisthoparia. 



Proetidffi, DicellocephalidEe, Lichidse, Acidaspidse, Bronteidse, 



Group 6. Gonatoparia. Suture cuts genal angle. 



Homalonotidce, Calymmenidas. 

 Group c. Proparia. 



Phacopidse, Cheiruridse, Encrinuridse. 



Jaekel, after a careful study of the Agnostidse, considers that these 

 Trilobites are sufficiently distinct from all others to be placed in 

 a separate division equal in value to that containing all other 

 Trilobites. He therefore proposes the name Miomera for those which, 

 like Agnostidse and Microdiscidse, have only three trunk segments, 

 and Polymera for all with six or more free segments. 



Beecher's Classification. 

 The developmental details upon which Beecher's classification is 

 based are too well known to justify recapitulation here. It will 

 suffice to recall the facts that in the earliest larval stages of both 

 opisthoparian and proparian Trilobites the free cheeks are not seen on 

 the dorsal side, but that, as development advances, the facial suture 

 appears first of all close to the margin and then moves towards the 

 glabella, so that the free cheek, at first narrow, broadens out. It is 

 therefore assumed — whether rightly or wrongly the future will decide 

 — that in the earliest stages the free cheeks are on the ventral side and 

 that the sutures run either along or below the margin. It is further 

 assumed that the occurrence of this hypothetical phase in so many 

 widely separated Trilobites is due to there having been a corresponding 

 stage in the evolution of both the Opisthoparia and the Proparia. 

 This leads naturally to the supposition that this hypoparian condition 

 might have been retained into adult life by some known Trilobites, 

 hence the creation of the division Hypoparia. "Whilst the divisions 

 Opisthoparia and Proparia are almost above reproach the same cannot 

 be said of the division Hypoparia. 1 Even Gurich to some extent 

 recognizes the two former, but rejects the latter. 



The Hypoparia. 

 In the English edition of Zittel's textbook the following families 

 are placed under the order Hypoparia: Agnostidse, Eodiscidse 

 (Microdiscidse), Shumardiidas, Harpedidse, Trinucleidse, Raphio- 

 phoridse. 



1 Cf. H. Woods, Cambridge Natural History, vol. Crustacea, ch. Trilobites, 

 pp. 226, 244. 



