Dr. G. Holm — On Baltoceras. 251 



II. — Baltoceras, a new genus of the Family Orthoceratid^. 

 By Gerhard Holm. 



ANEW Ceplialopodan species, Endoceras Burchardii, was de- 

 scribed in 1880 by Dewitz,^ who considered this species as 

 a very typical Endoceras. A description and diagram of its 

 ectosiphon is given in the introductory description of the siphon 

 of Endoceras as an example of a typical Endoceras-s\-phon. In 

 Endoceras Burchardii each septum, according to Dewitz," forms a 

 septal tube (funnel), extending backwards to the next septum, into 

 the septal tube (funnel) of which a small part of it enters. 



Eiidiger ^ has also described this species as an Endoceras, and 

 1 have myself, on the authority of Dewitz, occasionally mentioned 

 it under this name. Having had, however, many opportunities of 

 seeing it in the island of Oland, where it is pretty common in the 

 grey Lituites Limestone, I have long since been surprised to find 

 that detached siphons of it — without reference to its size or its 

 marginal position — far more resemble the siphon of an Ortlioeeras 

 than that of an Endoceras, not only as to shape but also as regards 

 substance and colour. Like a siphon of an Orilioceras, detached by 

 weathering, it never shows any septal fractures,^ but immediately 

 behind each septum there is a narrow, deep, groove-like contraction, 

 sharply defined in front as well as behind. In figures 2 and 2a, 

 taf. xvi, Dewitz, this can be seen very distinctly. Eiidiger, too, has 

 noticed this siphonal form, so exceptional in an Endoceras, and describes 

 it thus : " Die Erweiterung der Diiten erstreckt sich iiber den ganzen 

 Zwischenraum zweier Kammerscheidewande, so dass der Sipho nur 

 an der Durchbruchsstelle der letzteren eingeschniirt erscheint und, 

 isolirt, einer Eeihe Semmeln ahnelt." Nor does the weathered 

 siphon consist of a firm shell-substance, but always presents itself 

 more or less decomposed, the surface being of an earthy appearance, 

 like that of an Ortlioeeras. Having commenced some time ago to 

 prepare thin sections of a series of species in order to study 

 microscopically the siphons of the Lower Silurian Cephalopods, 

 I was deeply interested as I looked forward to the first section 

 of Endoceras Burchardii, and I was not astonished to find its 

 siphonal structure entirely different from that of Endoceras ; instead, 

 it was quite similar to the siphon of a species belonging to the 

 group Ellipochoanoida, Hyatt. 



The following illustrations. Figs. 1 and 2, drawn from sections, 

 show the structure of the ectosiphon. The septa, together with the 

 short septal tube (funnel), consist of two layers; one thicker in the 

 middle, of water-clear, colourless calc-spar, on both sides of which 



1 Dewitz, H., " TJeber eiuige ostpreussische Silurcephalopoden " : Zeitschr. 

 d. Deutschen geolog. Gesellschaft, vol. xxxii (1880), p. 391, taf. xvi, figs. 2, 2a. 



2 Ibid., p. 371, fig. 1, " Schematisclier Liingsschnitt in der Mittelebene." 



3 Eiidiger, H., " Ueber die Silur-Cepbalopoden aus den mecklenbnrgischen 

 Diluvialgeschieben," p. 29: Separatdr. aus Arch, der Freunde d. Naturg. 

 Meckl. 1891. 



* Holm, G., "Oni de endosifonala bildningarna bos familjen Endoceratidse " : 

 Gaol. Foren. Forh., Ed, xvii (1895), p. 607. 



