228 MICROOERAS. ANDRO<3YNOOERAS, KTC. 



Family LIPAKOCERATID.E. 



MICROCERAS. Abdomen flattened ; sides rounded or flattened. 

 The pilae in the adult are undivided upon the abdomen, and are 

 continuous with the large, single lateral pila?, which last may be 

 ornamented with either one or two rows of small tubercles or be 

 bare. The envelopment only covers the abdomen of each in- 

 ternal whorl, reaching no farther than the first row of tubercles ; 

 the umbilicus is consequently exposed in all the species. The 

 increase of the radii is slow ; the species have a greater number 

 of whorls than in succeeding genera, and are also of smaller 

 size. The septa are remarkable for their unequally divided lobes 

 and cells, the large size of the abdominal lobe, the insignificant 

 size of the two lateral lobes, especially the inferior lateral, and 



the great breadth of the cells. 



Lower and Middle Lias. 



M. BIFERUM, Quenst. T. 107, figs. 609, 610. 



ANDROGYNOCERAS. Sides of the adult whorl slope outward 

 and are ornamented with pilae, usually single and set with two 

 rows of tubercles. Abdomen narrow. The large pila; of the 

 young are split into smaller pila} on the abdomen of the adult, 

 but usually retain the characteristics of Microceras until a late 

 period of growth. The septa are more complicated than in 

 Microceras, and the increase by growth in the radii of the spiral 

 is much greater, the species consequently luive fewer whorls and 

 are of larger size. The envelopment mav cover up only the ab- 

 domen of each internal whorl, or extend over the whole side to 

 the internal line of tubercles. 



Middle Lias. 



A. HYBR.IDUM, Hyatt. T. 107, tigs. 616, 617. 



LiPAROCERAS. This genus differs from both of those previ- 

 ously described in the greater breadth of the abdomen, the 

 greater increase of the radii of the spiral, the consequently 

 smaller number of whorls, and the larger size of the species. 



Middle Lias. 



L. HENLEYI, Sowb. T. 107. tig. 618. 



Family DKROCEHATI D.E. (Includes the group Dorsati.) 



DKROCERAS. Whorls circular ; pilae depressed ; linear between 

 and bifurcated on the tubercles. Tubercles large, prominent, 



