672 ON DIPELTIS AND PROTOCARIS—SCHVCRERT. vol.xix. 



Type. — />. diplodiscus, Packard. 



Dipdtis (lifters conspicuously from Apus and LipUlurus in its nar- 

 rower head shield followed by two large tlioracic segments. If these 

 three parts in DipeUis were united into one, there would result a ce])h- 

 alon in general form very much like that of lApUhirns. The small 

 number of abdominal segments is another conspicuous diflerence as 

 compared with the recent genera. 



At first sight, Dipeltis might not be regarded as a member of the 

 Ai)odida', because of the two peculiar, and disproportionately large, seg 

 ments back of the cephalon. The presence, however, of longcercopoda 

 and a "vvide anal segment, the position of the eyes, and the A]>us-like 

 head shield are all characters of this family. The few and wide abdomi- 

 nal segments are not unlike those of LepidiirKs. On the other hand, 

 the very suggestive theory may be advanced that the cephalon and the 

 two large body segments of Dipeltis may represent a mature condition 

 of a stage in the development of Limulns, since the cephalothorax of 

 this genus is composed of seven segments, five belonging to the head 

 and two to the thorax. Against this view the fact may be urged that 

 no pre-Carboniferous members of the Hemiaspida', ancestors o\i Limn- 

 lus, show any near relationship to Dipeltis. The abdomen of the latter 

 has from seven to ten segments, and the anal i)iece with its two long 

 cercopoda is in complete harmony with the Apodidic and wholly unlike 

 Liniulus. 



The nature of the waters in which Dipeltis lived was marine. These 

 animal remains are found in nodules, associated with Prestn-ichia, 

 Eurypterus^ malacostracous, schizopod, and decapod crustaceans, 

 arachnids, insects, Aricniopectcn, and Solenoiiya, and numerous ferns. 

 This evidence indicates that the waters in which the rocks of Mazon 

 Creek were deposited were neither fresh nor brackisli, but marine 

 (])robably estuarine), and into these the plants and insects have been 

 blown or washed from the not far distant land. 



DIPELTIS DIPLODISCUS, Packard. 



(l^late LVIII, ligs. 2-5.) 



DipeWis (Jiplodi-sciis, rACKAKD, Anicr. Xat., 1885, p. 2d'S; Mem. >.'at. Afiul. 8ii., 

 Ill, Pt. 2, p. 145, ])!. V, figs. 2, 2a. 



Head shield twice or more than twice as broad as long. In the 

 adolescent stage, it is somewhat drawn out posteriorly at the sides, but 

 these extensions disappear with maturity. Ui)on the head shield there 

 is a more or less sharply elevated median carina, which is also present 

 on the two large thora(;ic segments. On each side of this ridge, imme- 

 diately within the anterior edge, are situated two small shallow pits, 

 w"hi(;h are interpreted as ocelli, and ])osterior to these are two faintly 

 preserved eye spots. Cei)halon nearly smooth with faint ridges, as 

 shown in Fig. 3. 



