Plate II. 



Deroceras planicosta. 



Fig. 1. Section of siphon and siphonal coecum. B, ovishell ; 6V, lining layer 

 of the shell ; d", median layer ; d'", external layer ; d, dorsal 

 layers of the young; le, first septum; 2e, second septum, and so 

 on ; R, siphonal coecum ; s', remnant of the horny layer of the 

 siphon ; s", accessory layer, lining the interior ; s'", sheath. 

 Magnified 317 diameters. 



" 2. Section at right angles with siphon * showing the form of the whorls. 

 ^Magnified 21.5 diameters. 



" 3. Centre of fig. 2 enlarged. D, shell; U, embryonal umbilicus. Magni- 

 fied 70 diameters. 



" 4. Segment of the siphon from Plate IV, Fig. 1, on first quarter of the 

 whorl ; D, shell on the abdominal side ; E, septum ; P, the 

 siphonal funnel ; P, siphonal collar. Magnified 158.5 diameters. 



" 5. Section of same specimen of nearly the same age. Amplification 

 same. 



" 6. Enlarged drawing of one of the sections of the siphon in Fig. 2 ; O 

 marks the bottom of the siphonal fissure ; J, the lateral wings 

 of the abdominal cell divided by the fissure, as seen from behind ; 

 F, the two truncated branches of the abdominal lobe. Magnified 

 104 diameters. 



" 7. Young, with nine first septa delineated, and the amount of the in- 

 volution of the first whorl around the ovisac is shown by the 

 shaded line reaching from the broken edge of the whorl. Mag- 

 nified 80 diameters. 

 All the specimens described above are from the same block of Wilt- 

 shire limestone as those given in Plate I, Fig. 1-8. 



Arxioceras semicostatum. 



Fig. 8. Side view of the young with eleven septa, magnified -f- 10 diame- 

 ters. F, ventral lobe ; H, dorsal lobe ; g', superior lateral lobe ; 

 h', superior dorsal lobe ; g", inferior lateral lobe ; k', superior 

 lateral cell ; 1', superior dorsal cell ; k", inferior lateral cell ; 1", 

 inferior dorsal cell. 



" 9. Side of three and a quarter whorls of an older specimen from the 

 same locality; magnified 21 diameters. 



" 10. Front view or projection of the same. O, siphonal fissure ; J, ab- 

 dominal cell ; F. abdominal lobe. 



ASTEROCERAS OBTUSUM. 



Fig. 11. Section of first two whorls and ovisac, magnified 49 diameters. 

 The left side only of this specimen is perfect. 



* This section is not exactly at right angles to the siphon, though the variation is 

 not perceptible except when Fig. 3 is drawn. Then the embryonal umbilicus is seen 

 to be absent from one side. This, however, is too slight a deflection to affect materially 

 the truthfulness of the general outline. 



