SPONDYLUS. 131 
the furrow on this valve is the introduction of a small intermediate rib between 
two main ribs on the left valve, so placed that at the margin of the valve the small 
furrow of the right valve fits into the small rib of the left valve. The number of 
the small intermediate ribs and the period at which they are introduced vary 
considerably in different specimens; in some cases they are quite absent, in others 
only a few occur, or they may appear in almost every groove. When these inter- 
mediate ribs are absent, the other ribs appear to be stronger and are equal in size, 
and separated by broad and deep grooves; the form named S. wqualis by Hébert 
belongs, I believe, to this group. Another form, with many intermediate ribs, was 
named S. duplicatus by Goldfuss ; this, however, has been regarded by most later 
writers (Geinitz, Reuss, d’Orbigny, Brauns, etc.) as inseparable from S. spiiosus. 
In the cases where the intermediate ribs are introduced at an early period, they 
become towards the margin of the shell almost or quite as large as the other ribs, 
so that the shell appears to bear a large number of rather small ribs, and differs 
greatly in appearance from the forms with no intermediate ribs. Between these 
different varieties every transition seems to occur, but certain types, as noted 
below, are more abundant in some zones than in others. 
The form described by Hébert as S. equalis' was found in the B. mucronata 
Chalk of Meudon. It was stated to differ from 8. spinosus, Sowerby, in haying 
(1) equal ribs (7. e. no intermediate ribs) on the left valve, and undivided ribs on 
the right valve; (2) the two valves of equal convexity; (8) spines on both valves. 
Hébert gave no figures, but Lundgren has figured specimens from Kopinge, which, 
however, do not show the spines. I believe that S. xqualis is only one of the 
varieties of S. spinosus. Forms found in the Upper Chalk of England (especially 
in the B. mucronata and Marsupites zones) agree in the first and second of the 
features above mentioned, and also in rare cases show spines on the left valve, 
although never so well developed as those on the right. This form, as already 
stated, passes into the one with many intermediate small ribs, It will, however, 
probably be convenient to refer to the two extremes as the xquualis-type and the 
duplicatus-type respectively. 
Plagiostoma brightoniensis, Mantell, seems to be an old individual belonging 
to the xqualis-type; it occurs chiefly in the M, cor-anguinum zone. Near the 
margin of the left valve intermediate ribs are introduced, and at this part on both 
valves growth-lines are well marked and close together, giving something of a 
frilled appearance. 
S. obesus, d’Orbigny,’ from the Senonian, appears to be only a variety of 
' It is recorded by Barrois from the B. mucronata zone of Studland Bay and Norwich. See 
«Rech. Terr. Crét. Supér.’ (1876), pp. 103, 163. 
9 
? «Pal. Frang. Terr. Crét.,’ vol. iii (1847), p. 675, pl. eccelxi, figs, 5—7. 
