. Bulletin. Natural history; Natuurlijke historie. 68 PEABODY MUSEUM BULLETIN 33. Fig. 10. Orientation of measurements made on pterioid-shaped species. L = Length W = width OL = oblique length IL = length from anterior extremity to base of posterior ear DML = length of dorsal margin H = height 1/2 = width of one valve AL = anterior length 0 = angle measuring prosoclinality The pattern of muscle insertion areas is closely analogous to that of pterioid genera such as Pinctada (see Newell, 1937, fig. 1). The basic units, the posterior adductor, the pallial line of small discrete insertions, the p
. Bulletin. Natural history; Natuurlijke historie. 68 PEABODY MUSEUM BULLETIN 33. Fig. 10. Orientation of measurements made on pterioid-shaped species. L = Length W = width OL = oblique length IL = length from anterior extremity to base of posterior ear DML = length of dorsal margin H = height 1/2 = width of one valve AL = anterior length 0 = angle measuring prosoclinality The pattern of muscle insertion areas is closely analogous to that of pterioid genera such as Pinctada (see Newell, 1937, fig. 1). The basic units, the posterior adductor, the pallial line of small discrete insertions, the presence of two strong pedal elevator inser- tions in the umbonal region, and one pedal retractor insertion closely associated with the adductor are similar. Minor differences are the variability in number and pattern of accessory insertions in the umbonal area and the occasional insertion pits on either side of the impressed posterior pedal elevator insertion area. The major difference is in the size and position of the pedal retractor insertion area. Most modern pterioid and isogno- monid genera have a curved or V-shaped adductor with a relatively large pedal insertion fused or discrete from it on the anterior side at about mid-height of the insertion area or between the arms of the adductor. Examination of specimens of the recent Isognomon isognomon (Linnaeus) in the YPM collections shows that although the position is constant the degree of fusion of the pedal insertion is variable. For most pterioid genera it is fused to the adductor. The pedal retractor insertion area of Pseudoptera siibtortuosa is relatively small, discrete, and situated at the anteroventral extremity of the adductor. The phylogenetic significance of this is at present unknown, but it would be possibly a simple step for the adductor to reflex and for the pedal insertion to migrate to a central position. A strong horizontal cross-ridging is present on the anterior part of some ad- ductor insertion areas and
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