JURASSIC OF THE PARIS BASIX. 



543 



Tkracia depressa (towards the top), Mytilus virgulinus, Ceromya 

 concentrica, and Pecten suprajurensis ; and to these Coquand adds 

 Ammonites longispinus, Pholadomya acuticosta, and Pleuromya 

 tellina. The rocks therefore which here underlie the " Portland " 

 are the same that have been hitherto recognized as Virgulian, and 

 represent the lower, or at most the middle, portion of our English 

 Kimmeridge Clay. AVe are therefore prepared to find that here also 

 the "Portland" beds are no higher than those usually so called. Xow, 

 tracing upwards the beds towards Angouleme, we find a set of rather 

 rubbly fossiliferous limestones, containing Pholadomya decemcos- 

 tata, Trigonia concentrica, T. cf. Bronnii, Area laura, and Cerithium 

 septemplicatum. These rocks become more sandy by degrees, and 

 at last form a peculiarly crisp calcareous sandstone. The fossils in 

 this sandstone, as noted, are Pseudomelania gigantea, Corbis Rathier- 

 iana, and Avicula Credneriana, all characteristic species of the 

 " Lower Portland." Xo more is seen in this section, as the Hip- 

 purite limestone lies unconformably upon the last-named stratum. 

 The higher beds of the district were therefore examined in the 

 neighbourhood of Jarnac. The section at Souillac could not be 

 verified as showing four subdivisions overlain by gypseous beds ; but 

 much oolitic stone is worked to the west of Jarnac, as far as Chassors 

 and Xercillac, to a thickness of at least 12 feet ; in this Cyprina 

 Brongniarti is the characteristic fossil. From the fact of this species 

 not being quoted, but in its place Cardium dissimile (only a single 

 doubtful example of which could be found), and from the similarity 

 of the casts of these two shells, is it possible to conclude that for 

 the latter we ought to read the former in Coquand's lists ? Still 

 something very like the Cardium dissimile was seen at Nercillac, and 

 also, as will be seen, in the He d'Oleron. The other fossils noted 

 at Chassors &c. are those often accompanying Cyprina Brongniarti, 

 or belonging to a higher zone, viz. Cyprina implicata, Astarte 

 rugosa, Pecten nudus ( = jarnacensis), Cardium Dufrenoycum, 

 Astarte regidaris, Corbida mosensis, and Mactra insidarum. Over- 

 lying these near Jarnac are more flaggy beds, with many small 

 oysters at the base. Comparing this with the more complete 

 development in the Haute-Hame, the beds here shown belong- 

 obviously to the zone of Cyprina Brongniarti; and as several of 

 the fossils indicate at least the upper part of it, though the 

 gypseous beds aoove give no clue to their age, and we are 

 therefore left without proof in this district of the equivalents 

 of the vacuolar oolite representing the true Portland, it is pro- 

 bable that such beds, though under a somewhat freshwater con- 

 dition, are actually reached, being represented by the so-called 

 " Purbecks." 



2. Department of the Lower Charente. — The rocks in this de- 

 partment, originally studied by d'Orbigny (14), have since been 

 more fully described by Manes (17), who divides them as 

 follows : — 



