34 ME. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [April 1 0. 1 4, 



less transverse, and in this respect resembles Productus ivallacei 

 Derby, figured by Chernyshev from the cor a and Schwagerina 

 horizons of the Urals. 1 A somewhat similar form, Pr. opuntia, 

 has also been described by Waagen from the Salt Range. 2 



Semireticulate Productids. 



If we except Productus cora and the ' spinulose ' Productids, the 

 remaining examples of the genus all belong to a semireticulate 

 group, in which Dr. Vaughan suggests three distinct forms can 

 be recognized. In the first the semireticulation is irregularly 

 developed and the spines are numerous, in this respect approaching 

 a scabriculate type of ornament. In addition to this character, 

 the steep sides of the shell and its general outline make it almost 

 impossible to separate it from a form which occurs commonly in 

 the uppermost beds of the Avonian in the British South- Western 

 Province, just below the Millstone Grit and also in the Middle Coal 

 Measures. It has been termed a ' scabriculocostate ' Productus. 



The shell figured by A. d'Orbigny 3 as Productus inca may also 

 be referred to this form, as it shows strong scabriculate characters, 

 and the semireticulation, in common with his other figures, is 

 doubtless considerably idealized. 



Salter refers Productus inca D'Orb. to Pr. semireticulatus 

 Mart., and his figure 4 possibly represents an example of this 

 species, although it certainly, as drawn, bears no resemblance to 

 the Pr. inca figured by A. d'Orbigny. 



The remaining two forms Dr. Vaughan considers to represent 

 late mutations of Productus antiquatus Sow. and Pr. semireticu- 

 latus Mart. The first shows the general shape and regular 

 reticulation of Sowerbys species ; but it exhibits a distinct senile 

 character in the way in which the radial ribs come together beyond 

 the reticulate portion of the shell — thus being considerably reduced in 

 number, and leaving relatively-broad flattened spaces between them. 

 At the points of junction the ribs frequently bear spine-bases. 



The specimens that I have determined as mutations of Pr. 

 semireticulatus Mart. (PL VIII, fig. 6) differ from the above in 

 the irregular nature of the reticulation and in the very pronounced 

 nodular character of the intersections between the ribs and the 

 concentric folds, in this respect being in a more advanced stage than 

 the typical Avonian forms and similar to those found in the Coal 

 Measures. The fusion of the ribs to form intervening flat 

 spaces on the skirts of the shell is also observed in this species, 

 and suggests a gradual passage to a more highly-specialized 

 semireticulate type with a completely-smooth margin, such as Pr. 

 oislinu from the Middle Productus Limestone of the Salt Range. 



1 Mem. Com. Geol. Russie, vol. xvi, No. 2 (1902) pi. xxx, fig. 8 & pi. lx, 

 figs. 19-23. 



2 Pal. Indica, ser. 13, vol. i (1887) p. 707 & pi. lxxix, figs. la-2b. 



3 ' Voyage dans l'Amerique Meridionale : Paleontologie ' 1842, p. 5 & pi. iv, 

 figs. 1-3. 



4 Q. J. G. S. vol. xvii (1861) pi. iv, fig. 1. 



