süBCLASS I TRILOBITA 699 



any part of the length, but is sometimes reduced to a short rudiment {Goldius, 

 Fig. 1340), or it may be even eritirely obscured (Nileus). The number of 

 axial Segments normally correspoiids to the number of pygidial, and varies 

 between two and twenty-eight. On the lateral lobes, all or at least a part of 

 the pleura may also be seen, being continued from the axis as ribs separated 

 by furrows. In these cases, the furrowed and the ribbed pleura can usually be 

 distinguished, but not infrequently they have entirely disappeared as surface 

 features. Many of the Cambrian Trilobites are conspicuous for their small 

 pygidium and elongated thorax. 



The outline of the pygidium is most frequently semicircular, parabolic or 

 elliptical ; more rarely it is triangulär or trapezoidal. The margin is entire, 

 less commonly dentate or spiny. The border, as in the case of the cephalon 

 and the pleura of the segments, has a reflexed margin, or doublure, which in 

 some genera attains considerable width. 



The Ventral Side. — The ventral side of Trilobites is commonly inaccessible 

 for purposes of Observation, since, as a rule, it is so firmly attaehed to the 

 rock that the organs, even though present, cannot be exposed by the ordinary 

 methods. Furthermore the appendages and ventral structures are so thin 

 and delicate that the most favourable conditions are necessary for their pre- 

 servation. For this reason, great uncertainty has prevailed regarding the 

 presence and character of the legs and various appendages. After a careful 

 preparation of their inferior side, by far the larger number of Trilobites show 

 only the vacant hollow space beneath the dorsal shell, and the hypostoma 

 attaehed to the reflexed margin of the cephalic shield. This common condition 

 of the fossils led Burmeister, in 1843, to the assumption that all organs on 

 the lower side, as in Phyllopods, were originally soft and fleshy. Previous 

 to this, however, Linnaeus, in 1759, described what appeared to be antennae, 

 and Eichwald, in 1825, announced both antennae and legs. Altogether the 

 early literature down to 1870 contains quite a number of claimants for this 

 discovery. Most of the evidence is manifestly ^'erroneous, and the two or 

 three cases which bear some semblance of validity are too obscure to be of 

 any scientific value. 



Billings, in 1870, published the description and figure of an unusually 

 well-preserved Isotelus gigas from the Trenton Limestone of Ottawa, Canada. 

 The ventral side of the specimen showed eight pairs of jointed legs on each 

 side of a median furrow. Soon after. Woodward described an antenna or 

 pediform cephalic appendage, lying beside the hypostoma of another individual 

 of the same species. Through the investigations of Walcott (1875-94) on 

 Ceraurus and Calymene, by means of transverse and longitudinal sections 

 of enroUed specimens, a number of problems have been settled as to the 

 characters of the ventral 



side. It is now known c .^^^^^^.^^^^ i 



that Trilobites possessed f , --.^..^i*"^'''^^''**' ' ^'^ ^*^ -"^^ 



a thin, external, ventral I \ m. ^ "^^^^^^^^^v 



membrane attaehed to the ^^^^ „ ,„,, 



, ^^^^ Fig. 1841. 



renexea margm OI tne Median vertical section of Ceraurus plpurexantliemus Green, c, 



cephalon thoracic SeßrmentS Coplialon witli hypostoma below ; tu, Moiith ; Vy Ventral membrane ; 



" .' ° ;, Intestinal canal ;;<(/, Pygidium (after Walcott). 



and pygidium. it was sup- 



ported by transverse processes which became thickened with age, and to 



these the legs were attaehed. 



