62 Thirty-first Report ox the State Museum. 



straightened out, while in others it is a closely-coiled spiral. At the bifurcation 

 the outer spiral springs from the base, which continues on to form the inner 

 spiral. The spirals are most ventricose at the center, tapering toward both ex- 

 tremities. Their curvature follows that of the dorsal shell. The ribbon or band 

 forming the spiral is slightly flattened and somewhat loosely coiled. In several 

 sections the spiral has been pulled out, so that the plane of the section passes 

 through it, as in fig. 5<? of Plate 1 Generally they present the appearance 

 of fig. 5/ of Plate 1, where the section appears to pass through the spiral in 

 its normal condition. A section cut obliquely from the posterior margin of the 

 glabella to below a line with the eyes in front, and between the dorsal shell and 

 hypostoma, shows one side of four pair of setiferous appendages. They appear to 

 consist of a short projection from the mass of calc-spar filling the cavity of the 

 glabella, to which a short joint is articulated. From the best preserved one, 

 eighteen setae radiate from the side and end of the outer joint. The setae aver- 

 age two mm. in length. A transverse section of the head of an Aoidaspis 

 Trenlonensis, from near the posterior margin across the posterior end of the 

 hypostoma, gives an opposite section of the setiferous appendage. It is essen- 

 tially the same as those in Calymene. It is situated beneath the dorsal shell, 

 on a line with the branchial appendages of the thorax, and is a modified con- 

 tinuation of the branchial series of appendages.* A narrow layer of calc-spar 

 connects the visceral cavity with the doublure of the head. A transverse sec- 

 tion of the head of a Calymene, cut so as to cross the hypostoma just within the 

 posterior end, shows the upper half of the space beneath the glabella to be filled 

 with calc-spar. This space is the continuation of the visceral cavity of the 

 thorax. From the lower lateral margin of each side, a jointed appendage 

 extends outward and downward. The basal joint is large — twice as long as 

 broad; from the lower slightly concave surface a slender joint projects, which is 

 followed by one, and, when the appendage is entire, by three or four joints. A 

 narrow band of spar unites the upper visceral space with the spar filling the 

 concavity of the hypostoma. Between the upper pair of appendages and the 

 glabella three pair of appendages project. Their basal joint is slender; and, in 

 two sections, closely resemble the maxillary joint of the leg of the Eurypterus, 

 modified in form, but undoubtedly subservient to the same use as a part of the 

 mouth. Two sections show three long, slender joints, but not a terminal one in 

 position. In three detached joints the terminal one is small and sharp-pointed. 

 In one beautiful transparent section the observer looks into what appears to be 

 the opening of the mouth. The basal joints of the manducatory legs have their 

 upper inner margins truncated, and are slightly convex ; four on each side, 

 forming an elongate opening above the hypostoma, the upper end of which is 

 closed by the approximation of the upper margins of the broad coxal joints of 

 the upper pair of appendages."}" 



Sections of Amphus platycephalm furnish evidence that it has axial append- 

 ages of essentially the same structure as those of Calymene and Ceraurus, as 

 the basal and second joints in the section are the same as seen in those genera, 

 and thft basal joint of the leg is attached in the same manner to the body. The 

 visceral cavity is attached to the shallow dorsal groove. 



From the above described and other sections, the following conclusions upon 

 the structure of the Trilobite may be given, as based upon the knowledge now at 



* These appendages are probably attached to the manducatory legs. The thoracic spiral 

 branchiae arc absent, or else simple projections. Sections, partially confirmatory of this, are at 

 hand. 



t Upper appendage is here used instead of posterior appendage, in the sense that the mouth, 

 in this species, was .situated between the ventral surface of the visceral cavity and the hypos- 

 toma. and opened posteriorly instead of directly downward. This would necessitate the curv- 

 ing <^"<i)c alimentary canal beneath the glabella to pass to the mouth. 



