SPONDYLUS. 131 



the farrow on tliis 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 ri])s are absent, the other ribs appear to be stronger and are equal in size, 

 and separated by broad and deep grooves ; the form named 8. sequalis by Hebert 

 belongs, I believe, to this group. Another form, with many intermediate ribs, was 

 named S. da'plicatus by Goldfuss ; this, however, has been regarded by most later 

 writers (Geinitz, Reuss, d'Orbigny, Brauns, etc.) as inseparable from *S'. .'^pinosus. 

 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 Hebert as *S^. aeqaalis^ was found in the 7i. mitcronata 

 Chalk of Meudon. It was stated to differ from S. sjjijwsns, Sowerby, in having 

 (1) equal ribs (i. e. no intermediate ribs) on the left valve, and undivided ribs on 

 the right valve ; (2) the two valves of equal convexity ; (3) spines on both valves. 

 Hebert gave no figures, but Lundgren has figured specimens from Kopinge, which, 

 however, do not show the spines. I believe that 8. sequalis is only one of the 

 varieties of 8. spinosns. 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 seqiialis-iy^e and tlie 

 duplicatas-tjipe respectively. 



Plagiostoma hrightoninnsis, Mantell, seems to be an old individual belonging 

 to the sequalis-tjpe ; 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. 



8. ohesus, 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. Crct. Super.' (1876), pp. 103, 163. 



2 ' Pal. Fran?. Terr. Cret.,' vol. iii (1847), p. 675, pi. cccclxi, figs. 5—7. 



