254 SYNOPTIC COLLECTION. 



nal shell preserved which exhibits a brilliant iridescence. 

 Where the shell is broken the septa and chambers are 

 visible. The middle portion of the septum in the upper 

 left hand specimen is seen to be nearly flat but the edges 

 are deeply fluted. In the largest specimen on the right, 

 the shell is mostly worn away, revealing the complex 

 character of the sutures. 



No. 611, Perisphinctes, belongs to another series in 

 which the shell is compressed, but the younger whorls 

 are not covered. The marginal siphuncle and the fluted 

 sutures are remarkably well seen in this specimen. An- 

 other species of this genus, No. 612, P. comptoni Pratt, 

 has long lateral processes or ears extending from the liv- 

 ing chamber. 



Phylloceras heterophyllum Suss. (No. 613). is not only 

 compressed but involute. The shell has mostly disap- 

 peared, revealing the extremely complex character of the 

 sutures. The section shows the protoconch in the cen- 

 ter, the closely coiled young shell, the septa, and the 

 chambers filled with mineral matter. As might be ex- 

 pected these extremely specialized Ammonites are not 

 found in the ancient Palaeozoic strata but occur in the 

 Mesozoic formations. 



Aturia zizac Sow. is found in the recent Tertiary rocks, 

 and is a compressed, involute form with specialized 

 sutures, and differentiated, nearly dorsal siphuncle (No. 

 614; also PI. 615, figs. 1-3). Growth in this genus is 

 very rapid during the nepionic stage, the coiling is close, 

 and the funnels of the siphuncle are flaring (see fig. i). 

 Fig. 2 is a front view, and fig. 3 a side view of the nepi- 

 onic and half of the neanic stages. Fig. 2 shows the 

 nearly dorsal position of the siphuncle, and fig. 3 the 

 lobes of the first four sutures. No. 616, Aturia aturi 

 Bast, is a valuable specimen obtained by breaking down 

 the shell so that the funnels of the siphuncle are exposed. 



It has been shown that the Nautiloids and Ammonoids 

 both arose from straight Orthoceras-like shells, and in 



