OF PLANORBIS AT STEINHEIM. 45 



It will be observed, however, that, in the two holes sunk in the Old Pit, under my 

 directions, the pieces of Jura limestone began to be thrown up as soon as the pick entered 

 the dark clay layer. Dr. Hilgendorf, in none of his sections, found tiie dark clay layer, 

 which forms the base of these two sections. Nevertheless, No. 2, in his First Section, 

 consisted above of clay layers containing PL tenuis, and below the large ano-ular 

 fragments of Jura limestone, which show that he had here reached the Iwttom of 

 the deposits. It seems as if no other inference was possible, since everywhere this 

 indicates the same fact, and is accepted as the limit. 



I cannot synchronize his sections and any of mine with success, as in feet mio-ht 

 be anticipated from the great variation which occurs often within a few feet between 

 the details of the stratification. 



His second section, given on p. 480-81, is the most complete of all, since here he 

 obtained his PI. m. Steinhewiensis. With relation to this there are some statements 

 which have confused me in searching for the exact situation of this form. All of his sec- 

 tions described are in the Old Pit, and only a general description is given of one in the 

 New Pit, and of the " dritte Grube " which is the Cloister Pit — the East Pit he does not 

 mention. In the New Pit he distinctly states that he did not reach the Steinheiynensis 

 layer; and in the Cloister Pit he saw the Steinheimensis zone, but not "in situ," ("nicht 

 in situ gesehen.") 



This narrows the places in which this zone was first observed to one section, his 

 second section. 



On p. 492 in his account of PI. m. Steinheimensis, and its genetic series, he writes as 

 follows : " Die Zone ( 1 ), deren Schilderung den Anfang machen miisste, ist mir leider nur 

 mamjelhaft hekannt, da sie bei meinem letzteu Besuch der Gegend unzuganglich war. 

 Sie enthiilt die echten Steinheimenfiis ; doch zeigen schon hier einzelne Exemplare eine 

 Andeutung von tenuis — Kennzeichen ; am Ende der letzte Windung tritt oben eine 

 undeutliche Furche auf, und die Abplattung der Umgange ist auf der Oberseite oft schon 

 recht merklich. Doch kein einziges Exemplar wiirde mit einem tenuis verwechselt wer- 

 den ktinnen. Dagegen kommt in der nachsten Zone, (der uatersten, zu der ich in der 

 alien Grube (jelamjte), ein ;e/i;«t.s-artiges Aiissere den meisten Exemplaren zu, nur die 

 scharfe Kante auf der Unterseite ist bei keinem Exemplare ausgebildet, so dass ein 

 eigentlicher tenuis (unter etwa 1000 Exemplaren) noch nicht zu linden ist. Diese 

 erscheint erst in der nachsten Schicht in der Begleitung von echten, meist involuteren 

 Steinheimensis ; beide Zonen zusammen geben eine Reihe von Exemplaren die von der 

 schonsten Runduug eines Steinheimensis zu der breiten scharf kanntigen Form des 

 tenuis jede Absttifung zeigen, Doch scheint in den tenuis Zonen schon eine kleine 

 Liicke zu sein, so dass aus ihr allein eine Reihe nicht mehr gut herstelljjar ist. Man kann 

 die 3 Schichten als Steinheimensis-zone, Uebergangszone des Steinheimensis zu tenuis, 

 and teuuis-'ione bezeichnen."' 



In this description he declares the second zone to be the lowest which he reached 

 in the Old Pit, although the fii'st zone is put down in his second section. This 

 discrepancy however, disappears, when taken in conjunction with the fact that he 

 describes zones 1 and 2 in the section, as two layers of shells contained in one 



1 The italics arc all my own, except, of course, the uames of the species. 



