426 INFERIOR OOLITE AMMONITES. 



Sonninia subirregularis, S. Buchman. Plate XCVIII, figs. 4, 5 ; and young form, 



Plate LXXVII, figs. 6—9, Plate 

 LXXXVIII, fig. 4. 



Discoidal, compressed, hollow-carinate. Whorls ornamented with regular 

 spines up to about 47 mm. diameter of shell, with costas and spinicostas as 2 to 2, 

 roughly in pairs, to 85 mm. diameter, and as 3 to 1 to 130 mm. diameter; 

 lastly, with regular, fairly well-marked, close, direct, somewhat reclining, ventrally- 

 inclined costae. Ventral area rounded, divided by a well-defined, not large 

 hollow-carina. Inner margin well defined, subconvex, nearly upright. Inclusion 

 one-fourth, not up to spines after 43 mm. diameter. Umbilicus graduate, spinous 

 in middle. Whorl-section oblongo-quadrate. Suture-line with fairly ornate lobes ; 

 L with intra-axial, anisosceloid, inequicellate terminal lobule ; anisometric lateral 

 lobules, the outer bipartite. 



In ornament and proportions this species stands between Sonn. irregularis 

 and Sonn. marginata ; but it is less coarsely ornamented, and costate more 

 regularly and at a much earlier age than the first ; while it is more spinous, 

 and spinous for a much longer time than the second. Except for its septation 

 this species might seem to connect these forms ; but it has a more ornate 

 suture-line than irregularis, and a considerably more florid septation than 

 marginata. The suture-line agrees with that of S. crassispinata, the outer lobule 

 of L being slightly more bipartite. From 8. acanthodes the more upright ribs are 

 an easily noted feature of distinction. 



The small specimen illustrated in PI. LXXVII, figs. 6, 7, is presumably the 

 young of this form. The only noticeable difference is a slightly more abbreviated 

 inner lateral lobule of L (PI. LXXXVIII, fig. 4) than the young of 8. subirregularis 

 would be expected to possess. 1 The younger specimens (PI. LXXVII, figs. 8, 9) 

 may be classed with this one. 



Sonn. subirregularis occurs in the Goncavum-zone of Bradford Abbas. The 

 type-form is illustrated in PI. XCVIII : fig. 4, the side view, a copy from a 

 photograph, much reduced ; fig. 5, the suture-line. The dimensions of the type 

 are — radius 162 mm., umbilicus 114 mm., umbilicus, one whorl back, 49 mm., two 

 whorls back, 22 mm., three whorls back, 95 mm. Breadth of whorl, from inner- 

 marginal suture to carina, 93 mm. Thickness of whorl, about the middle, 62 mm., 

 over costae, 68 mm., at top of inner margin, 57 mm., all without test. 



1 Id the specific identification of young specimens the smallest differences should be noticed with 

 rigid exactness ; by the time these specimens had become adult small initial differences might have 

 grown so pronounced as to entirely alter the appearance of the shell. 



