228 MH'KOCKKAS. A M > KO( , V NoCKIl A S. KTC. 



Kamily 



Abdomen flattened: sides rounded or flattened. 

 Tlii' pihe in tin- :ulult are undivided upon the abdomen, and are 

 continuous with tin- large. single lateral pihe. 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 iirst row of tubercle*: 

 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. BIFERLM. (Juelist. T. 107. figs. COD. Oil). 



A M>uo<;yNo<'KKAs. Sides of the adult whorl slope outward 

 and are ornamented with pihe. usually single and set with two 

 rows of tubercles. Abdomen narrow. The large pihe of the 

 younii' arc split into smaller pihe on the abdomen of the adult. 

 but usually retain t he characters! ies of J//r/v,rr/7/s until a late 

 period of growth. The septa are more complicated than in 

 Microcerdi*. and the increase by growth in the radii of the spiral 

 is much greater, the species consequently have fewer whorls and 

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

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

 the internal line of tubercles. 



Middle /,f<n. 



A. nviiiMi.r.M. Hyatt. T. 107. ligs. (ill',, r.17. 



Li I- \KIHKK.\S. This genus differs from both of those previ- 

 ously described in the greater breadth of the abdomen, the 

 -realer iiici'case of the radii of the spiral, the consequently 

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



Middle Lias. 



L. II KM, KYI. Sowb. T. 1(17. fig. I'.IS. 



Family I ) K K ( X ' K K A T I I > . K. (Includes the group Dorsati.) 



i'l.i;", BBA8. Whoi'ls circular ; pihe depressed ; lineal' between 

 and bifurcated on the tubercles. Tubercles large, prominent. 



