I too prefer magical beast. And if we need to solidify that selection...
We're obviously going to have to stat up Psychic Killer Clams as well.
No, I'm not joking.
Still, we'd better finish the Carnivorous Scallops first.
I'll increase the Dexterity to 5 since they're mobile. That gives them an Armor Class equivalent to the original's AC4.
It occurs to me that I'd like them to be immune to mind-affecting powers, either as a legacy of their Vermin origins or due to Nascent Psychic Powers.
Also gave them scent, since it says they have special organs to detect waterborne chemicals.
Rather than my original Improved Grab/Constrict/Swallow Whole idea I'm going to try to fold it all into one special attack, I've called it "Trap Shell" but I don't care for the name that much - can you think of anything better?
I've removed the "Giant clams have no visual organs" bit from Blinsight, since the AD&D Giant Clam had lots of little eyes to see with, just like real world
Scallops whose eyes can detect light and motion.
So, here's a first stab at it:
CLAM, GIANT, CARNIVOROUS
Large Magical Beast (Aquatic)
Hit Dice: 4d10+8 (30 hp)
Initiative: -3
Speed: 5 ft. (1 square), swim 15 ft.
Armor Class: 16 (-1 size, -3 Dex, +10 natural), touch 6, flat-footed 16
Base Attack/Grapple: +4/+13
Attack: Ram +8 melee (1d10+5)
Full Attack: Ram +8 melee (1d10+5)
Space/Reach: 10 ft./5 ft.
Special Attacks: Paralyzing cloud, trap shell
Special Qualities: Blindsight 30 ft., camouflage, immunity to mind-affecting effects, scent
Saves: Fort +6, Ref +1, Will +1
Abilities: Str 20, Dex 5, Con 15, Int 1, Wis 10, Cha 9
Skills: 7
Feats: 2
Environment: Warm aquatic
Organization: Solitary or cluster (2-10)
Challenge Rating: 2
Treasure: 50% standard
Alignment: Always neutral
Advancement: 5–8 HD (Large); 9-12 HD (Huge)
Level Adjustment: —
Blindsight (Ex): Giant carnivorous clams can ascertain all foes within 30 feet using vibration.
Camouflage (Ex): The rough shell of a giant carnivorous clam is usually draped with barnacles, anemones, and bits of coral, which help it to blend in with its environment. Creatures must succeed on a DC 20 Spot check to notice a giant clam. Anyone with ranks in Survival or Knowledge (any water- or sea related skill) can use one of those skills instead of Spot to notice the giant clam.
Paralyzing Cloud (Ex): Once per day, a giant carnivorous clam can release a cloud of chemicals in a 10-foot spread (15-foot for a Huge clam). All living creatures within this cloud must succeed on a DC 14 Fortitude save or be paralyzed for
1d4 rounds. Creatures with immunity to poison or paralysis are unaffected, and creatures resistant to poison or paralysis receive their normal bonus on their saving throws. A
delay poison, neutralize poison or
remove paralysis spell removes the effect from a paralyzed creature. The save DC is Constitution-based.
Trap Shell (Ex): As a standard action, a giant carnivorous clam can attempt to trap any creature within reach up to one size larger than itself. The target can make an attack of opportunity against the clam, but if it does so it is not entitled to a saving throw. An opponent that does not attempt an attack of opportunity must succeed on a DC 17 Reflex save or be trapped. Immobilized creatures, including those affected by the clam's paralyzing cloud, are not entitled to a saving throw. The save DC is Strength-based.
If the trapped opponent is smaller than the clam, they are drawn inside the clam's stomach, where they take 1d4+5 bludgeoning damage plus 1d4 acid damage per round. If the opponent is the same size as the clam or one size larger, the clam uses the razor-sharp edges of its shell to saw at the trapped opponent, automatically inflicting 1d6+7 points of bludgeoning and slashing damage per round.
A giant carnivorous clam can be forced to release a trapped opponent by succeeding at an opposed Strength check against the clam's Strength check (the clam gets a +4 racial bonus on this Strength check). A trapped victim can also try to cut its way free with any light slashing or piercing weapon, by doing
[15?] hit points of damage to the clam's AC 15 interior.
