CALYMENE AND CERAURUS. 47 



Walcott's views on certain portions of the anatomy can best be set forth in the form 

 of a few extracts (1881, pp. 199-208) : 



The Ventral Membrane. In those longitudinal sections in which the ventral membrane is most perfectly 

 preserved^ it is shown to have been a thin, delicate pellicle or membrane, strengthened in each segment by a 

 transverse arch, to which the appendages were attached. These arches appear as flat bands separated by a 

 thin connecting membrane, somewhat as the arches in the ventral surface of some of the Macrouran 

 Decapods. . . . 



In by far the greater number of sections, both transverse and longitudinal, the evidence of the former 

 presence of an exterior membrane, protecting the contents of the visceral cavity, rests on the fact that the 

 sections show a definite boundary line between the white calcspar, filling the space formerly occupied by the 

 viscera, and the dark limestone matrix. Even the thickened arches are rarely seen. 



The mode of attachment of the leg to the ventral surface is shown [in transverse and longitudinal 

 sections of Ceraurus and Calymene}. These illustrations are considered as showing that the point of articula- 

 tion was a small,- round process projecting from the posterior surface of the large basal joint, and articulating 

 in the ventral arch somewhat as the legs of some of the Isopods articulate with the arches in the ventral 

 membrane. The arches of the ventral membrane in the trilobite . . . afford a correspondingly firm basis for 

 the attachment of the legs. 



Branchial appendages. The branchiae have required more time and labor to determine their true structure 

 than any of the appendages yet discovered. They were first regarded as small tubes arranged side by side, 

 like the teeth in a rake; then as setiferous appendages, and finally as elongate ribbon-like spirals and bands 

 attached to the side of the thoracic cavity, the epipodite being a so-called branchial arm. All of these parts 

 are now known to belong to the respiratory system, but from their somewhat complex structure, and the 

 various curious forms assumed by the parts when broken up and distorted, it was a long time before their 

 relations were determined. 



The respiratory system is formed of two series of appendages, as found beneath the thorax. The first 

 is a series of branchiae attached to the basal joints of the legs, and the second, the branchial arms, or epipodites. 



The branchix, as found in Calymene, Ceraurus. and Addaspis, have three forms. In the first they 

 bifurcate a short distance from the attachment to the basal joint of the leg, and extend outward and downward 

 as two simple, slender tubes, or ribbon-like filaments. In the second form they bifurcate in the same mariner, 

 but the two branches are spirals. These two forms occur in the same individual but, as a rule, the more 

 simple ribbon-like branchia is found in the smaller or younger specimens, and the spiral form in the adult. 

 . . . The spiral branchiae of Ceraurus are usually larger and coarser than those of Calymene. 



The third type of the branchiae [consists of rather long straight ribbons arranged in a digitate manner 

 on a broad basal joint]. As far as yet known, this is confined to the anterior segments of the thorax. 



The epipodite or branchial arm was attached to the basal joints of the thoracic legs and formed of two 

 or more joints. This has been called a branchial arm, not that it carried a branchia, but on account of its 

 relation to the respiratory system. It is regarded as an arm or paddle, that, kept in constant motion, produced 

 a current of water circulating among the branchiae gathered close beneath the dorsal shell. . . . 



Of the modification the respiratory apparatus underwent beneath the pygidium, we have no evidence. 



In his latest publication (1918, pp. 147-153, pis. 26-28, 33), Walcott has reviewed his 

 earlier work on Calymene and Ceraurus, and presented a new restoration of the former. 

 The coxopodites are now interpreted as being similar to those of Triarthrus and Neolenus, 

 but the exopodites are still held to be spiral and the setiferous organs labelled as epipo- 

 dites rather than exopodites. 



COMPARISON OF THE APPENDAGES OF CALYMENE AND CERAURUS WITH THOSE OF 



TRIARTHRUS. 



As one may see by reading the above quotations from Doctor Walcott's descriptions, 

 he found certain branchial organs in Ceraurus and Calymene which have not been found 

 in other trilobites but otherwise the essential features of the appendages of all are in agree- 

 ment. 



