HAPLOPLEUUOCERAS. 
299 
biologically, it is the form which has retrogressed the most. It is also latest 
geologically. Soiverbyi-zone, Toulon. Only one specimen recorded. 
The genealogy of the Zurcherise is shown in Table VI. Both branches 
illustrate retrogression from a bispinous, through an unispinous, to a costate stage. 
SoNNiNiN^ {continued). 
Genus — Haplopleueoceras,^ 8. Buchnan. 
{Type — Haplopletteoceeas subspiitatum:, S. Buckman.) 
Discoidal, compressed, carinate and sulcate. Whorls ornamented with direct, 
ventrally-inclined ribs, upon which are two spines, one towards the inner, the other 
at the outer edge of the lateral area. Ventral area carinate, more or less 
bisulcate, also ribbed and striate, the carina, sometimes very slightly crenulate, 
cutting through the apex of the V-shaped ribs and striae. Suture-line : superior 
lateral lobe trifurcate ; superior lateral saddle deeper than other saddles ; inferior 
lateral lobe lopsided. 
When I wrote my paper on the ' Descent of Sonninia, &c.,'^ I stated (p. 657) 
that suhspinatum^' was descended from " Pleuroceras" hawskerense. The 
similarity between the two forms is remarkable ; but I noticed at the time that one 
fact did not quite fit in with this idea of descent — the whorls of " subspi7iatum,^' 
though more compressed than those of "hawskerense,'^ were not more included. 
Another solution of the genealogic problem of " suhspinatum " presents itself ; 
and, if it be correct, it discloses a most remarkable instance of homoplastic 
development. The species Zurcheria pugnax (Vacek) (p. 298) has two rows of 
spines, and a very rudimentary carina. The appearance of carina and sulci on 
the ventral area would practically change "pugiiax" into suhspinatum and it 
may be noticed that during progression the development of an incipiently carinate 
ventral area is, as the Arietidse show, as follows : — rudimentary carina, carina with 
flattened area each side, the flattened areas becoming more and more depressed to 
form sulci, finally a sulcate-carinate ventral area. 
If this idea of descent be correct, then two branches arose from " pugnax ;" one 
progressive, leading to " subspinatum the other retrogressive, Zurcheria. 
The geological position of " subspinatum " accords with this idea ; and its life 
history does not seem to show anything against it. The inner whorls are 
unispinous at a very early age (PI. XLIX, fig. 5), and the second row of spines is 
acquired during infancy (PI. XLIX, fig. 6). Owing to the crystalline condition 
^ airXdos, single ; TrXevpa, rib ; Kepus, horn. 
2 ' Quart. Journ. Geol. Soc.,' vol. xlv. 
