1883.] 311 [Hyatt. 



crease in proportion to the deeper involution of the whorls, as in 

 some Nautiloids. Agon, fecundus, sp. Barr. Syst. Sil. pi. 11, is 

 less involute, and shows how closely the larval shells repeat the 

 adult peculiarities of the parent gyroceran form, Mimoceras. In 

 fig. 4, occurs the extraordinary variety with straight larval apex, 

 which leaves us in no doubt, that these shells must have had a re- 

 mote ancestor with a straight cone in the adult stage. Agon, fide- 

 lis and verna sp. Barr. pis. 8, 9 and Vanuxemi, Hall, exhibit spe- 

 cies with the highly concentrated development common in the 

 more involute, and compressed shells which generally terminate 

 the series we have studied. They have skipped the larval pecul- 

 iarities of the gyroceran stage, and become close coiled and even 

 involute on the first or second whorh Agon, tabuloides, sp. Barr. 

 pis. 4, 244, exhibits in the section figured a decided annular lobe, 

 showing that this may occur in some Silurian species of the Ammo- 

 noids, though usually a Devonian characteristic of both Ammonoids 

 and Nautiloids. Another curious fact is that it has the internal 

 depression we have called the cone, which among the Kautiloidea 

 is not in our experience fully developed until after the Carbon- 

 iferous, and certainly must have been rare, if it occurred in other 

 forms in the Paleozoic. 



Pinnacites, Mojsis. Med. Trias Prov. p. 181, was merely 

 mentioned by that author and the type given, as the Pin. emacia- 

 tns sp. Barr. of the Silurian. We have not examined the type, 

 but in the Mus. Comp. Zool. there exist fine specimens of the De- 

 vonian form which does not differ apparently from its Silurian ally. 

 The highly compressed and acute whorl becomes excessively invo- 

 lute at an early age, and we were able to follow the sutures far 

 enough to see, that in the larva the natural decrease of involution 

 must bring about the disappearance of the umbilical lobes, and 

 reduce the sutures to the outlines of Agoniatites. The abdomen 

 is also broader in the young and the whorl in section is identical 

 with the adults of Agoniatites, and these facts indicate direct de- 

 rivation from the latter. The septa of the adults are double con- 

 cave in correspondence with the lateral lobes, the internal surface 

 being divided by ridges corresponding to the lateral saddles. 

 There is a broad dorsal lobe, with two small, widely separated 

 dorsal saddles, the impressed zone is very deep, and there is no 

 annular lobe, unless the broad median lobe may be so considered 



