1883.] 309 [Hyatt 



The young are asellate, the first suture having no ventral saddle, 

 according to Branco. The larvae in some forms are cylindrical 

 and open whorled, but in all the higher forms closed, and, though 

 still asellate, have the broad form of the ammonitic embryo. 



Mimoceras l includes two well known species of true Goniatites, 

 but these are separable from Bactrites in no essential characteris- 

 tic, except the presence of a permanent protoconch upon the apex. 

 The septa have simple concave lateral sutures, and dorsal saddles 

 without annular lobes, the whorls have no impressed zone, and 

 the shells are, therefore, not really truly nautilian in form, but 

 gyroceran ; and even the compressed whorls are similar to those of 

 some Bactrites. The ventral lobe is a simple funnel lobe, as in 

 Bactrites, and it divides the ventral saddles in the same way. 

 Their characteristics, and the protoconch ally them, however, even 

 more closely with Anarcestes and oblige us to place them in 

 the Nautilinidae. This evidence appears to need but one more link 

 the finding of a Bactrites with a globular protoconch. Type, 

 Mimoc. (Gon.) compressum, sp. Beyr. Sandb. Verst. Nass. pi. 11, 

 fig. 4 ; also, Mim. (Gon.) ambigena, sp. Barr. Syst. Sil. pi. 3, 12, 

 and possibly Gon. Dannenbergi, Beyr. Verst. d. Bay. Rheins, 

 Ueberg. pi. 1, fig. 5. 



Anarcestes, Mo j sis. Mediterr. Trias Pro v. p. 181, was pointed 

 out by that author but insufficiently defined, and a list of species 

 given in a note, the only characteristic cited being the living 

 chambers, which are said to be long. The genus is also character- 

 izable by the broad semilunar whorl, the abdomen broader than 

 the dorsum, this peculiar form is present in the later larval stages, 

 and is maintained even in excess in some very involute species in 

 which the abdomen in consequence becomes excessively broad, 

 the sides very narrow, and the umbilicus very deep. There are 

 some discoidal species, like Anar. crispus sp. Barr. pi. 9, fig. 31, 

 with rounded whorls until a late stage of growth, but most of the 

 species depart from the tubular outline at a very early age. 

 Sandberger shows that in one variety of Anar. subnautilinus 

 the first whorl is gyroceran, and Branco, in the same species, 

 demonstrates, that variety vittiger is close coiled. Compressed 

 whorls occur in some Silurian species, such as Anar. neglectus, 



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1 Mtjwsi a mimic. 



